Have you forgotten to mention the SNP, they are the third biggest party at Westminster now wiping Labour out in Scotland and the Lib Dems as well ?
Surely this had more of an impact than UKIP or the Greens ?
Not really, you have to remember the size of parties bares no relation to the number of votes they had.
If every SNP voter had voted Labour, we'd still have a Tory slim majority government with 329 seats, (288 Lab), i.e no change from now
If every UKIP voter had voted Labour, we'd have a Labour minority government with 299 seats (Tory 267), perhaps a strong Lab/SNP coalition with 355.
If every UKIP voter had voted Tory, we'd have a massive Tory majority government (402 seats vs 165 Labour)
If every Green had voted Labour we'd have a Tory minority government (320 seats, vs 244 Labour)
If Every Green, SNP and Plaid vote went to Labour, you'd have a Tory minority (316 seats v 293 seats)
If every LibDem had voted labour, there'd likely be a Labour/SNP coalition, Tory plurality of 298 seats, but unlikely to pick up the needed 27 other seats. Labour on 274, SNP resilient on 54 so a total of 328, just squeaking into a majority.
So I retract my green statement, it's all down to what happens with the UKIP and lib dem votes in the next election.