No catch. Confirmed no traffic management, no download limit, free router, free migration. This is purely for customer retention.
I'm 800m away from the exchange and get 12.2Mbits down and 1.1Mbits up.
I pull an average of 120Gb/month over the line with no problems.
Edit: just to add I was paying £35.50/month with O2 before which is crazy amounts.