Never heard of Naims but they seem to be an Entanet reseller of which there are many more well known, they all have the same packages and the same pricing.
Fast, Idnet, adsl24, aquiss, titanadsl and so on. Bit of concern with Entanent at the moment and some resellers are considering choosing a different wholesale supplier.
I have 3 pcs in the house, 2 kids playing xboxlive and youtube etc. and me downloading the odd linux distro and 30 gigs is just about enough so you should be ok. With Entanet weekends don't count towards your allowance.
Plusnet is owned by BT but are totally seperate as far as pricing etc., good deals but long contracts now I think (Entanet resellers usually 1 month).
Check out your exchange at samknows site (sometimes the mods here delete links) for LLU operator presence
such as Be/O2, great deals via quidco and if you have an O2 mobile sim (£70 cashback for o2 via quidco).