Rossian,
Hmm.... I've seen all the tricks in the salesman's book.....
Look behind the brand, find out :
(a) Are they just reselling BT's product like 99.9% of the ISPs out there, or are they reselling a LLU product, in which case, which one ? I can guarantee you one thing, its not going to be their own LLU network !
(b) Probe them a bit on the post sales support. Who is really providing it, and where are they located ?
If they are cagey about releasing details on the above, then give the product a wide berth.
With a brand name like John Lewis, one would hope they've done their homework and chosen to partner with a reasonable ISP. But in todays bottom line driven world, you can never be too sure, Waitrose, for example, is not what it used to be, standards are slipping a bit there !
Edit to add:
Actually, scrap the above (I've left it there as a good lesson for all though), look at the T&Cs .... line one :
Plusnet plc is the service provider under these Terms and Conditions and by agreeing to these terms and conditions you (the customer) will be contracting with us (Plusnet plc)
So you're buying a Plusnet service, Plusnet are owned by BT, so in all probability they're using BT's network.
If you still fancy it (not making a recommendation !), just contract with Plusnet directly, no need to waste time with the John Lewis branding. Although kudos to John Lewis for making it reasonably easy to find the answer !
Personally, if you're going to go through the hassle of switching, I would look at LLU rather than BT ! But that's my opinion.