> being a multi-party democracy using a FPTP electoral system, which tends to boost
> the representation of leading parties at the expense of smaller parties.
More precisely, FPTP tends to boost the representation of leading national _and local_ parties (the latter being best exemplified by the SNP) at the expense of smaller national parties. Small local parties are usually toast anyway even in proportional systems due to thresholds.
During the 2015 elections the system worked exactly as designed, by favoring parties that are strong in one or more nations of the UK at the expense of parties that are a bit meh in all of them.