If your commute is mainly reasonable tarmac, I would try and stretch to a Specialized Sirrus. 2008 model is currently retailing for just less than £300 with
Evans Cycles - reduced by over £50.
It's a lighter bike with tyres slightly narrower than the bikes you have already looked (28mm) at, but not so narrow to be uncomfortable or impractical. You will feel much more nimble and agile underneath you once you get used to the handling and it be much less effort to propel.
I know it's £100 over budget but it will make a huge difference.
If however you are going to be cycling on trails or uneven circumstances, then your current choices are probably okay.