I don't think I've heard of any cabinet or exchange that has congested FTTC service. There *might* be, but I don't recall any obvious signs.
The ISP is a separate story - and Plusnet *say* they will be keeping their pipes sufficiently sized to not cause congestion to their unlimited packages.
With a Virgin number, you won't get estimates from the BT Wholesale checker
, but you should be able to get something from the postcode and/or address checkers instead.
And just because there is a BT socket on the premises doesn't mean there is still a copper connection all the way back to the exchange, or that it is plugged into anything - BT could have reused any part of the line or equipment in the meantime.
A stopped service is still connected and gets dialtone, and is cheapest to reconnect. A ceased service has been physically disconnected, and the resources freed up for other customers.
If the old line has been unused for a while, it is likely to be ceased. This needs a larger restoration, and costs more - provided there are still enough resources available. An engineer in the exchange building will need to wire the E-side line (which may be different from the one used before) to a linecard (via a DSLAM if you went for plain broadband as a simultaneous provide), and a different engineer will turn up at your property (and probably the cabinet) to connect and check the service works all the way through. An engineer is certainly required at the cabinet when FTTC is provisioned onto the line.
IIRC Plusnet charge the same £50ish fee for reconnection of a ceased line as for provision of an entirely new line. It is only the reconnection of a stopped line that is cheap. Take a look at this old PN forum page
, though obviously the T+Cs and charges can be different nowadays.