Indeed you have to take FTTP for gigabit vouchers, and I don't believe I said otherwise. I said "subject to the terms of the vouchers", and "a basic 40M service" (FTTP implied, since FTTP is what we are talking about).
Plusnet don't sell FTTP, but there are several others who do.
The cheapest I know about currently is Sky at £25 for 80M, but it's not available in all exchanges. BT Fibre 1 (50M) for £28 is sure to be available. Aquiss will do you 40M/2M FTTP for £30 (discounted to £15 for first 3 months, 12 month contract, so averages out to £26.25 per month).
Anyway, all I was saying is to remind people that even these are "gigabit vouchers", they are not committed to taking a gigabit service. Otherwise they may baulk at the ~£60 per month cost.