£25 per month generally won't get you a 300GB anytime allowance, that much is certain and those that claim to offer the impossible ("unlimited" or super high data allowances etc.) usually end up suffering from service or support problems.
Namesco and the various Entanet vISP’s (e.g. Aquiss) will give you a good allowance for £25 and if you expect a good service in return then that’s probably the best you can get. However if you’re willing to sacrifice support and service quality then some providers like Tiscali and Fast4 claim to give you an almost all you can eat package but you might find yourself with complaints.
Regarding binary newsgroups, most broadband ISP’s seem unusually coy about the degree of usenet access given to their customers so you’ll have to ask them directly before subscribing. In addition you should be aware that many providers will stick a separate data cap on newsgroup access in the small print.