You've not stated whether you want a pay-as-you-go phone or one on a contract. However, people who put the stress on 'cheap' usually prefer p-a-y-g.
I use a Nokia 1100 (p-a-y-g on Vodafone) and I''m perfectly happy with it. It's simple to use and the text is fairly easy to read. (It's not particularly big but it's certainly clear enough for me. I'm one of the many people who ought to use bi-focal glasses but have never got round to getting any. I have to squint at small text on a page but I can read the phone display with no problems). The phone cost me just under �25 from Asda.
None of that will be of direct use to you, because the Nokia 1100 has been discontinued. However, it's successor, the Nokia 1112, is basically the same. It's available, on Virgin pre-pay, for �21.99 (with free delivery) from Amazon:
http://www.amazon.co.uk/Nokia-1112-Prepay-Mobi le-Virgin/dp/B000MKWLJG/ref=sr_1_1/202-5998648 -5055854?ie=UTF8&s=electronics&qid=1176992522& sr=1-1
Chris