The standard mail and parcels trains made use of the dedicated Royal Mail terminals just as the TPOs did. So Sheildmuir near Glasgow, Low Fell on Tyneside, Doncaster, Warrington, Rugby(?) and Willesden. Probably amongst many others, but thats the ones I can remember.
The network was extensive: From what I saw in north east England, it seemed to be primarily based around a Scotland and North East England to the Midlands and South West corridor which made use of the ECML, and another Scotland to London corridor via the WCML.
Parcels trains also called at intermediate stations where mailbags were loaded from the platform: In the mid to late nineties, I can remember this taking place at Carlisle and York. Staffords Royal Mail terminal was a platform alongside the passenger station, and at Bristol Temple Meads, the terminus for a number of mail trains from various places, the trains were unloaded in the station and there are a couple of sidings on the south side of the station that held quite a substantial number of mail vans during the day.
The Royal Mail class 325 postal units were introduced in 1994 for general parcels traffic, and I think were concentrated on the WCML Sheildmuir and Warrington to Willesden parcels trains.
On the ECML, class 90s were used on Sheildmuir/Millerhill and Low Fell to Willesden parcels trains, which were fewer in number than on the West Coast I think, and some of these were TPOs.
For some examples of services, there used to be a 14:07 Low Fell to Bristol Temple Meads, which usually only loaded to about five or six vans, and was always used as a balancing move: In the mid nineties, this featured a double headed pair of RES Class 47/7s, which later changed to top and tail class 67s, and inbetween these periods I can remember seeing a class 86 used as far as Doncaster, with a 67 dead on the rear.
There was also a much lengthier train of around 11 vans that passed through Newcastle southbound at around 17:45 from Sheildmuir, electric hauled by a class 90 or 86, which may have conveyed TPO traffic, think it may have continued onwards to Willesden, but Im not sure.
An afternoon Sheildmuir to Willesden parcels train, formed of two class 325 Electric Postal Units, called at Carlise station to pick up mail at about 15:30.
Thats my rough recollections, hope they are of some help, Im sure others will be able to offer more detailed information.