For clarity, if you are served by PCP5 out of Totty, then it is outside 467 Bolton Rd. This is a neighbouring patch to myself, and I've often grafted on this patch when workstacks dictate.
As B*Cat has alluded to, the dB loss is quite high at 26dB. It's not so much the line-length from PCP to premises, as it is the size and type of the cable make-up. Alas, many D-side cables (PCP to Pole/Underground DP), are singular feeds, ie: unlike E-side cables (Exchange to PCP), we very rarely have the opportunity to swap you onto a better, lower loss, cable.
You are at the higher end at 26dB, but one thing I do know is that the 'Guesstimator' underestimates the predicted speeds by some margin, plus the 'sub-15Meg' rulings have been removed a while back. Basically, try and find a provider who is willing to give the installation a go and see if you're happy with the outcome ??
Sorry for quick response, will have a look on again later.