Combined modem/routers for VDSL2 are a bit thin on the ground at the moment, and I'm not sure how many support BT's future requirements (eg vectoring).
If you want anything fancy in the way of router facilities you may also have to part with quite a lot of money!
The big advantages with separate units are that you can choose a router that does what you want it to do, and that the modem belongs to BT. So if it goes belly up or gets fried in a lightning strike, they just replace it. Though a spare is handy to have (cheap enough on eBay) to cover the few days that takes.
Some people say that the separate units take up too much space… an attitude I could understand if the modem looked like a computer tower case, but it doesn't. Afaic the only downside of a separate modem is that it takes up yet another 13A socket.
Fwiw I use the OpenReach modem (Huawei ) with an Asus RT-N66U, it does everyhing I want and more.