Hi Steve, MPD are a subsidiary of the online white goods retailer AO.com, so not likely todisappear overnight. Looks like a similar setup to Carphone Warehouse, but online only. Full contracts, SIM free, SIM only. I see they don't offer any deals on EE, but that's irrelevant to you. The virtual carriers they offer, Smarty and VOXI, run on 3 and Vodafone respectively - you know the pros and cons of this setup, so I'm not going to dig that one up again.
Many of the poor Trustpilot reviews are the fault of their trade-in partner or people using a cash-back website first. Seems all their cusomer service people are called Lauren. But their Trustpilot rating is much better than CPW, though I've used the latter half a dozen times and never had a problem.