Again, the same point: if you left the job where you used customer data then how do you deal with that data in your memory? Either you don't give any reason to believe you used it, least you're getting charged with espionage and this would include not working for the competition, effective putting you under indefinite unpaid non-compete. Or you don't perform your job duties, and this gives no reason for someone to pay you.
If it's really like this in UK, I can see how you can think non-competes are not needed. I'd rather have an American non-compete where I get money and a defined term though.
I think there's a big difference between "data" and understanding principles of something. If you knowingly use "data" that your old employer hasn't made public for the benefit of your new employer, then you are probably not acting ethically. If your new employer is interested in you because of the things you know that they don't have access too, you are definitely not acting ethically. If you happen to know something that gives you an advantage, you can probably make advantage of it without disclosing that proprietary information to your new employer. Example: you now know that all avenues of trying to do X don't produce results. If you want to do Y at the new company, you can just not waste your time doing X, but you don't need to say why, and suggest trying a different approach. If someone else insists on doing X, you could still say that you think it's a bad idea, and suggest general reasons why, but probably you shouldn't say "No, at Blah we tried variants of X many times and it never worked." You can't unlearn the things you know, but you can handle that information ethically. Generally people have a pretty good read if what they're doing is ethical or not.
Whether you're in breach of a non-compete would ideally be exactly the same as whether you acted unethically or not. The purpose of enforcing the non-compete is to try to prevent the situation happening. In the UK, we rarely have non-competes and where we do, it'd have to go to a court to determine if you acted unethically or not (assuming honouring the contract is ethical) and attempting to prove if you did or not and/or the damage caused if you didn't.
But like, I say, there's a big difference between "data" (which you seem concerned about) and something you know/learned (which seems to be what your argument is predicated on). Using "data" would be corporate espionage, using something you learned because you understand the concept wouldn't be.
> effective putting you under indefinite unpaid non-compete
Not really, because most people do the decent thing, and there is generally no reason to suspect otherwise, and so these things never even go to court. If the company is convinced you have appropriated trade secrets, it probably will.
But people aren't under "indefinite unpaid non-compete" because most people just do the decent thing, and so most people are assumed to do the decent thing, and people just start their new job and get on with life. Non-compete agreements going to court almost never happens.
But it's important to add that I think this issue of trade secrets is different to non-compete, because years after you leave a company, if you leaked proprietary information of theirs that is still important in giving them a commercial advantage, you should expect to get sued even if you'd had a period of obeying a non-compete.
If the clients are already customers of the new company, then he can make them offers which he knows they would find tempting. There'd be nothing to suggest he ever did anything untoward, and even if a case was made, it'd be thrown out of court pretty quickly.
If the clients weren't already customers of the new company, but as soon as he joined they were approached, then there's a good case to be made that his actions were taken using commercially sensitive information. There's a good chance the court case would find against him. If it could be shown that other people cold called those clients, maybe phoning their public reception phone number and asking who they should contact, that's good evidence that there was no misuse of commercially sensitive information. If the new company suddenly knew exactly who to contact in procurement for the clients, and had phone numbers and e-mails, it's basically a slam-dunk.
If the prices they offer to the clients are in-line with the prices they offer other clients, there's not much of a case to be made. If the client is receiving unusually high levels of discount that are slightly better than what the old employer gave, it's pretty clear.
Like I say, in the UK, in general it is assumed that when someone moves to a new company, there will be some level of customer loss if there is a particularly good relationship between the employee and the client. That kind of stuff can easily be dismissed in court as a genuine existing close relationship (or maybe even the client knew them on linked-in and saw they'd changed to a competitor and approach them for a better deal) and almost no company would even bother trying to pursue this kind of case. In the opposite case, where it's very clear an employee had "stolen" a customer list, they definitely would pursue it because there'd be a high probability of winning that case.
Sadly, it's impossible to give a clear cut answer without talking about ethics. If you conduct yourself honourably and do the decent thing, there's close to zero chance someone would sue you for moving to a competitor, and even if they tried, it's almost certain they would lose the case. If you do the opposite, it's very likely they will sue you and also very likely they will win the case.
There's no magic line to try to skirt close to. But it's also not, as you seem to think, some kind of perpetual hold over a previous employee for people in the UK.
I also think the situation in the US is pretty similar. It doesn't really matter if you'd obeyed a year long non-compete, if you then gave year-old information to a competitor firm while it was still commercially sensitive, you could still be sued and have a good chance of being found at fault.
