How long is a piece of string?
I guess, on average, that any repeatable congestion doesn't come from BT's core network, nor from their pool of connections between the core and their ISP equipment. If it did, every BT user would be reporting the same kind of problem.
If it happens to you regularly, then it suggests something closer to your particular exchange or your particular cabinet - in which case swapping to a new ISP may not have an effect.
If it happens every day, with repeatable restrictions, it is probably personal - perhaps something set on your account.
It may be best if you take this problem over to BT direct, rather than have us answer generically. I think BT-Care staff look in on the BT forum here (they
have an ID, but don't post that much), or you can look at their own community website (see the "help" link contained in the profile of the ID I just linked to).
But do let us know how it goes.