I would ask them what their contention ratio in your exchange is if you are thinking of them.
Don't waste your time doing that, particularly if you are only planning to sign up for a cheap residential product.
Not only will they probably be reluctant to tell you the answer, but also ISPs don't care much for residential customers, they are at the bottom of the foodchain. You get what you pay for.
If you want lower contention on DSL, you have to sign up for a business DSL service with someone who can offer you at least a contractual 5:1 commitment.
Contention occurs at more places than just the local exchange. Don't forget about the wonders of traffic shaping either.
If they are just reselling BT's whoesale DSL implementation then they are at the mercy of BT and BT no longer formally define specific contention ratios, they define acceptable performance ranges.
UPDATE:
For Plusnet, see :
Plusnet Broadband Download Speeds and
Plusnet Traffic Management