I have run my own router with those from sky, bt, now (which seems identical to sky's). Don't believe any isp router stops you using your own, as long as you use yours as a secondary router (i.e. keeping isp router as is, just connecting the wan port of your own router to one of the ports free at your isp's router - as long as their IP ranges, ie first 3 numbers of the ip address, are not the same, which is generally the case but easy to change if not).
There are many advantages in keeping both routers going: now you have two sharing workload (e.g. wifi/nas/printer etc), or you can dedicate your own router to vpn traffic, or you can give both routers the same ssid but placed in different locations thereby extending your wifi range, etc. etc.
Do make them occupy different wifi channels though.