The only new bike I'd really consider actually under £300 is this: http://www.decathlon.co.uk/triban-300-road-bike-white-id_8239800.html
If you hang around and you're lucky there might eventually be another deal on something like the Merlin PR7, but I wouldn't hold your breath. You can do a lot better for 300-500, otherwise definitely consider looking for something used.
it's definitely worth stretching to the Triban 500SE at £330 imo
http://www.decathlon.co.uk/triban-500-se-road-bike-black-id_8306187.html