|I take it the the OP is using the phone line surge protector that is incorporated into a lot of UPS designed for home use.|
If you use a cable that an RG11 plug at one and and a BT style plug on the other or a BT to RG11 adaptor then there should be no problem. Connect that from the wall socket to the UPS and a standard phone cable with RG11 at either end from UPS to modem. The only time it will effect your DSL speed is if you have a crappy cable with lose connections in the plug.
The BT phone socket has three wires coming out. The third wire is the ring signal that is extracted within the socket and not within the telephone as the rest of the world does. This third wire is not needed for DSL and is probably blocked by the DSL filter. It is needed if you connect to a dial-up modem and want to use auto answer.
Consequently dial-up modems made for the UK market are wired slightly different than the rest of the world and you should always use the cable or BT-RG11 adaptor that came with the modem.