Assuming I am flying straight at another aircraft with mode c, at 2500 ft, what sort of range would I typically pick up the other contact?
Your own height has very little to do with it - other than to the extent of the limitations in the PCAS user manual - which is well worth a read and on the link I gave.
As long as the other aircrafts transponder is being interogated by either a ground based radar or an airborn system (such as Sywatch or TCAS) PCAS will read the squitter. That means that cover for most of the UK is pretty solid.
You will be warned of the traffic at over 5 nm. At 5 nm at typical GA v glider speeds head on you will have over 2 minutes to deal with the conflict.
I suspect when you hear the usual - limited radar service due to range and / or radar clutter that does not mean the response of PCAS will be reduced unless there is terrain between you and the radar head.