Not a bad idea that! though to be honest in parts of Derbyshire you would possibly struggle with 42/25 especially if you're not a really strong cyclist...it's not called the 'Peak District' for nothing!
Sorry - my geography isn't the best! Sure, 42/25 still requires a good turn of the pedals on the bigger hills. The steepest I have here is 11%. But it's still a manageable ratio and most double chainsets are 52 42 at the front anyway - so unless you ride a triple or a compact, most people will just need to get used to 42/25, or 42/28 etc as their easiest gear.
I didn't notice the info on your rear cassette in the OP. If you have 52/17 as the best option at the minute even 42/17 would still be challenging in hilly areas (though still much better). Maybe the best way to get a good ratio, as chambo34 suggests, would be to change the cassette, but I'd still recommend changing to a 42 single to see how that goes in the interim (rather than the hassle and cost of installing a double chainset, front mech and shifters etc). They would also look more original (IMO) with a single front chainring.
The worst that can happen is that you find you miss a bigger gear on the front, and at that stage you could take the plunge into a double chainset.
PS an example of a 42 though not sure if its the right BCD for you:http://www.ebay.co.uk/itm/Stronglight-4 ... 20d49bc25c