I think the Tories would struggle in 2015. In OTL, they could offload much of the blame for austerity and other problems onto the Lib Dems, whose vote they could subsequently steal at the next election to get themselves into majority. Not only would that not be the case, but the absence of a moderating influence would mean they press ahead with some of there more unpopular proposals of OTL, for instances dropping the top rate of tax to 40p, and pressing harder with austerity.
I can't see UKIP's rise not taking place, but the continued existence of the Lib Dems as the anti establishment party (who now have a popular high profile leader after the debates) would limit them somewhat, and they probably wouldn't end up with any MPs. Maybe Cameron would be pressed into promising a referendum by the right of the party in response, but given his precarious majority, I can't see it being till after the next election.
The consequences of a stronger Lib Dem presence could also mean no SNP majority in 2011, and thus, no IndyRef, and no SNP dominance of Scottish seats at Westminster. Like UKIP, there rise would still happen, but be more limited. The charges of Labour being in pocket of the SNP will be less potent. However, Clegg will also attract a number of voters from both parties, seeing as he would likely be considerably more popular than both of the other party leaders. I would predict a tight race, with Labour and Tories more or less even, and the Lib Dems picking up 70-80 seats or so. I would lean toward them opting to work with Labour in this scenario, they, and particularly, Miliband were more open to electoral reform than the Tories, and they wouldn't be offering an EU referendum either.