I use an XDA2i - it is a superb piece of equipment. I can access weather websites using GPRS connection (costs about £1 for every 1meg downloaded).
Website I use is
www.avbrief.co.uk where I can get TAFs, METARs and F214/215.
To get METARs and TAFs and F214/215 takes about 80k of bandwidth in total.
In contrast and using the fancy cumbersome Met Office site it takes about 250k to get the same data !
I've had an XDA/XDA2/XDA2i in it's different guises for about 4 years now and swear by it.
O2 used to be dreadful, but I stayed with them on the strength of the XDA ! I have to say their customer service is getting better.
You would probably need a minimum 12 month contract, £30 a month would get you 200 free minutes of airtime, and you could probably get an XDA2i for about £200 - if you haggle you might get more.
Or ....if you are already with O2, ask to speak to the 'retention' team - play hard to get and say you are changing provider ... see what you can squeeze out of them by way of an upgrade.
Or ..... you could buy an unlatched XDA2 off eBay for about £110 and stick with your current provider ..... (ps might have unlatched XDA2 going cheap in next two weeks
)