Here's what their terms and conditions say...
"The 'Fee for booking your Theory Test' is the combined DSA / DVTA Theory Test Fee (Currently £31.00) and the TheoryTest.net administration fee of £20.00 (including VAT at current rate)."
So they're charging you over fifty quid to book a theory test.
If you book your tests (both theory and practical) from this page: http://direct.gov.uk/en/Motoring/Motoringtransactions/DG_066356 it will cost you just the £31 fee.
But perhaps it's quicker? Or more straightforward?
No! It's not! Booking your theory test takes perhaps 10 minutes. It takes about the same amount of time to do it via this booking agent. So they're charging you £120/hour, at least, to do absolutely nothing, and to provide yo with a service that you can get for absolutely nothing via other channels.
Mug's game, innit.