It is a matter of preference and convenience when it comes to a two-box setup. Power usage would be slightly higher (but only slightly) if you are using both a separate router and modem as opposed to an all-in-one.
If you are looking for a modem/router combo that supports VDSL, it may be worth looking at the Billion BiPAC 8800NL. It will set you back about £70, but it also supports ADSL/2/2+, has SNR tweaking (not hidden like on the 7800N!) and has the same Broadcom chipset as the HG612 (I think, but could be wrong!) - the modem which is supplied with FTTC broadband.
The only real drawback is the wireless as the antennas are internal, and not external, so it may hinder usage at a distance. The fact that the 8800NL only has one 1GbE, and the rest 100Mb, may also be a drawback if you are using multiple devices via Ethernet.
It may be worth checking the spec to ensure future compatibility with upcoming standards if that matters to you. As of current, widely used standards are supported.
If you happen to be signing up for BT Infinity, I believe you should be supplied with a Home Hub 5, which has an integrated modem that supports VDSL. Although, from what I have read, the HH5 suffers from stability issues (disconnections, line errors, loss of speed) which is probably due to the fact it is using a Lantiq chipset.
I am not sure about a separate router as there are a lot of different routers you could combine with an Openreach Modem. Is it all about the features you need, and how much you would be willing to spend.