So which ISP are you with, and have you a long contract remaining ?
I ask, because I know for your area, Sky and TalkTalk are possibilities (but TalkTalk repeatedly gets complaints of slow speeds and billing problems)... If you are close to the end of your contract, there's a specialist ISP (not cheap, must say, but you'd perhaps only need them for 1-2 months) called AAISP (LINK)) {If you can limit how much you use, then something like 2 GB of daytime use and 50 GB off-peak, would cost you £20 (+VAT) each month.
Reason I mention them is because their main Broadband page (HERE) says
"We'll fix your line even if you are with another ISP!
If you are migrating your service to us, even though you know you have a problem with your line, we'll take on the fault. We'll tackle the problem and get it fixed within one month. If we don't then you can migrate away and owe us nothing for your migration to us and your service charges for that month"
Whether you can migrate to them and try their service for a month or two depends on who you use at present, how many months are left, etc. Best if you are with an ISP which uses BT / Openreach (eg BT, rather than TalkTalk or Sky)
The AAISP broadband packages are far from the cheapest but might get the problems solved for you. They have a 30 day notice on their "Mix and Match" account... and, big warning - if you accidentally picked the "Home::1" account that has a 6 month minimum contract, at £25 (+VAT)
I think you might do OK to try them out for a month or two, see if they can improve your speed, and then perhaps switch to Plus.Net. Since you are in the country, you have fewer choices, and higher prices than the cheapest offers going round, but hope this is helpful and any line fault(s) can be sorted for you.