The services center around mapping GPS information to the UK national coordinate systems, specifically ETRS89, OSGB36, and ODN from WGS84, the primary GPS coordinate system. The key benefit is that the resultant coordinates match exactly what you would expect to find on ground-surveyed maps and take into account all of the peculiarities of the GPS network.
For example, the RINEX data server provides information surveyed from precisely-located points in the UK that watch the GPS signal drift and record that information. If data from a dual-frequency (such as receivers that compare L1 & L2c) GPS is used, then accuracy of up to 1cm can be achieved with moderate observation times (1hr).
Beyond this, they are also posting a database of 900+ passive GPS stations that can be used as marking points when correcting GPS data.
Finally, there is an online coordinate transformation package that translates between ETRS89 and OSGB36.
Nice features, available for free.