Have you asked BT to do a line test? They will do it while you wait. I had very variable speeds recently (Up to 8 Mbps contract) and speeds fell to about 0.6. The ISP were reluctant to admit it but it turned out to be to do with the LLU connection in the local exchange. Now getting 8 - 9 Mbps.Worth keeping a log of speeds to discuss with ISP.
OnePlus 5T review: Hands-on