The Triban 3 is the bargain of the century at the moment as it is only £250 for a well spec'd beginners roadbike. If I had a bit more space and a more understanding wife then both my kids would have one.
Second hand is a good option if you know what you are looking at and are not afraid of some light fettling.
I bought my oldest son a used Renolds 531 early 90's Raleigh in tip top condition for a shade under £50 off
ebay. He has been doing some 30+ mile rides on it in the last few weeks in training for the Manchester2Blackpool ride this summer and loves it. All original parts except for a ghastly saddle and carbon seatpost which went back on ebay and sold bringing the cost down to a tickle over £30.
My reason for buying this bike is that it is the same as the one my older brother had when I was a teenager but it goes to show that you can get an awful lot of bike for not very much money if you know what to look for. The equivalent bike in todays market would easily be £500-600 new.