I've checked, and your line connects at around 5Mbit/s - which it's done reliably for a while and is in-line with what the line should be capable of.
It would be good to get a sense of what speeds you're getting when you have problems - so I'd recommend running a speed test.
Given the line performance is good the best thing to do is look for local issues that could be affecting performance - reboot your router try a different browser or update the one you're using if it's not fully up-to-date. If you have more than one machine check on both - if one is slow then that would indicate the issue is with that computer, rather than the connection.
On the separate point of what speed your line is capable of - you are on an exchange that will support ADSL2+ - and your line should get between 7 and 12 Mbit/s according to the estimates. It also supports FTTC (superfast broadband) and the speed estimate for that is 69Mbit/s, so quite a jump. Before considering switching though, I really think checking for issues on your computer(s) that could be affecting speed would be worthwhile.
ZeN Performance and Process Improvement Manager