It's certainly more diverse than the USA, but saying 15% is misleading. There are 650 seats. Labor and Conservative hold 559, 2 are vacant, leaving 89 held by other parties (~13.5%). Out of that, 54 are held by the Scottish Nationalist Party, but the fact is Scotland is basically a 2-party system between SNP and Labor, so it doesn't represent "the system" working differently rather than a politico-cultural divide between Scotland and England. Plaid Cymru represent Wales with 3 seats, and they're also the second-largest party in the Welsh Assembly after Labor, so that means Wales is basically a 2-party system between Plaid Cymru / Labor similar to Scotland.
SDLP+Sin Fein+Ulster Unionist Party+Democratic Unionist Party represent Northern Ireland with 17 national seats. So it's more diverse, right? Nope. There are 4 parties because of sectarian divides. Two parties are fighting it out for the catholics, and two other parties are fighting it out for the protestants. It used to be almost all UUP and SDLP, but those guys lost most of their seats to DUP and Sin Fein after the peace accords. So it's gearing up to be a 2-party system between DUP and Sin Fein. The funny thing is that the parties fear their own side more than the other side: DUP+Sin Fein have formed a coalition government representing Protestant+Catholics, and there's a rival coalition of the other Protestant party & other Catholic party. So it's like a double-layered two-party system where people refuse to vote for a party lead by the other religion.
So we should exclude these regional parties which represent their own micro-level-two-party systems. That leaves UKIP, LibDem, Green and Independent members as representing true diversity. Which adds up to 14 seats, or 2% being not some variation of a regional two-party divide.