You have to use the SuperHub on Virgin, it's paired to your account and nothing you can currently do will alter that. Sky use MER but it's not true MER, you only need an old BTO HG612 or ECI modem and if your chosen router will run DDWRT it can easily be made to work with Sky's MER implementation. Once you accept that then you're looking at 'dumb' routers without a modem and with 3rd party firmware support.
Personally I find consumer grade wifi on routers tends to suck. With that in mind wifi was not a priority for me aas i'd be running some Ubiquiti AP's to take care of wifi. Like you I wanted 3rd party firmware support, I needed compatibility with Sky's MER implementation and personally I wanted a dual core ARM CPU over a MIPS as the market is shifting to ARM. Asus offered the AC68U and AC56U with the same CPU/RAM/ROM but the AC68U had better wifi and cost more, I went with the AC56U as the wifi was not going to get used on the router anyway.
If I switch ISP my old HG612 BTO fiber modem will do ADSL/ADSL2+ and VDSL or I can set the Super Hub to modem only mode, in effect it will work with any ISP.