I would say £300 gets you a usable bike, but expect to have to repair / upgrade fairly quickly. I would go second hand, or stretch to £500 and try and get a good deal.
My friend bought a £300/400 Halfords road bike recently as he had signed up for an event. He's really enjoyed it and it has got him into cycling. I serviced it for him recently and I would have some doubts over the durability. It also wasn't very well set up, with bizarre cable routing that really hindered the brakes and a few other niggles. The things I thought were good about it:
- Gets you out for a low cost
- Reasonable (usable) weight
- Set up like a proper road bike
- Double chainset. What on earth the majority of people buying a £300 road bike want with a double chainset is beyond me.
- Some strange low rent shimano parts. You will need a spanner and socket set to assemble, put it that way.
- Poor quality tyres
- Wheels significantly out of true after 6 weeks use