Go for the £7.50 plan until they, if ever, catch you.
The £7.50 plan is not MEANT to be used for modem use, streaming, etc. However nothing is blocked. I know this as I'm streming radio1 right now on my £7.50 web and walk plan, and I've also used my
n95 as a modem before and it works fine.
I think what they are really hinting at is the data usage you use. As in if you were on the £7.50 plan and were streaming and downloading like mad they would call you up on it. As long as you keep within the 1gig fair use I honistly don't think they care that much what you use it on.
Regards,
Simon