A tip for getting one free eye test (and a bit of a plug for the company I work for) - if you join the Boots Health Club (in association with the Ad Card), you get a voucher for a free eye test. It takes a few weeks to arrive, but if you are in need of a test soon, that's a way to get it free. Also, if you use the Advantage Point machines in the larger stores, you can often print a voucher that gives you a free eye test when you buy a complete set of frames.
I'm not sure about the rest of the country, but in the store I work in, Boots eye tests are only £18, not £25. I think they are £20 in Specsavers, but I recently got a test done there for only £5 on a special offer.
Both my parents are entitled to free eye tests as my mum works at a computer all day and my dad's prescription is really strong (-10!), but my brother and I aren't - even though we're both in full-time education, we're also both over 21, so we don't qualify for free tests at the moment.