Also if buy and switch to a new router.
Will I be able to easily switch between the two routers (for testing) or will I always have to call up BT to tell them which router I'm using so it works for the internet?
The router you use is of no interest to BT and nothing needs to be configured by them. You can switch away to your heart's content but be very careful of upsetting DLM. Interruptions to your service by swapping routers around makes it look as though you have an unstable line which DLM attempts to stabilise by reducing your speed. Do not switch between devices any more than once or twice a day if you want to maintain speed.