Sometimes you have a working broadband connection, but it doesn't seem to be very fast or responsive... what do you do? Well, the first thing to try is a "Speed Test", using speedtest.net. You should do a number of speed tests, over a few days and at different times of day, to see if there are times when the connection is better or worse. Keep a record of all the test results, including dates and times. If the test results are only poor during evenings then your connection is probably just being throttled by your provider due to excessive demand across their network. If it's consistently slow, there are other tests to run.
BT Wholesale is the place to go for a detailed test of your connection. To do this you need to plug a computer (using an ethernet cable) directly into the primary router of the premises. If you're on a Fibre connection (eg, FTTC) then you need to set up a special network connection for this - as described in this Plusnet article on creating a PPPoE dialler. Once you've set up your wired connection, go to the BT Wholesale Broadband Performance Test and follow the instructions there.
If your speed test results still show degraded performance, then you will need to raise a support ticket with your broadband provider. Give them the results from your BT performance tests, and they should then be able to get BT onto the case!