To answer a few questions and add a little more detail on the QinetiQ Alpha Jets and Boscombe Down aircraft:
The PA31 is used as a comms aircraft to ferry test teams to and from off base meetings and trials. The Harvard, as has already been mentioned, is used as a photocase aircraft for parachute trials by the C-130, and by ETPS to demonstrate the stalling and lateral-directional flying qualities of a 60 year old aircraft that has a moderately large radial engine. Also, many of the ETPS students have never flown a taildragger so it exposes them to real flying!
Soiled Glove has raised some interesting points but is a little off track on some aspects. In the late '90s ETPS needed some more airframes to supplement its fleet, Fast Jet Test Squadron at Boscombe Down needed some more aircraft for continuation traning and Llanbedr needed some aircraft for chase. DERA evaluated several options and chose to buy some ex-GAF Alpha Jets to fill these requirements. ETPS did not specifically acquire the Alpha Jet because of its spin characteristics. In fact, the GAF Alpha Jets were not cleared for spinning in service as it was not a requirement for the close air support role in which they were used. Currently, there is not a spinning clearance for the QinetiQ Alpha Jets although there is a project underway to clear the ETPS aircraft to spin. The FAF Alpha Jets are very similar and are cleared for spinning. They have basically 3 spin modes. The normal erect mode is moderately oscillatory as would be expected for an aircraft with such a mass distribution. However, it can then be driven into a flat spin mode with an "eyeballs out" longitudinal acceleration of up to 3g. This is the really fun mode and I know of no other swept wing jet that is cleared to be put into a flat spin intentionally. The third spin mode is an inverted spin which is reasonably conventional for this type of aircraft. Overall, the Hunter was a more interesting aircraft to spin than the Alpha Jet except for the flat spin mode.
The Alpha Jet is definitely superior to the Hawk T1 in ACT. It has much better sustained turn performance, better unloaded acceleration, better roll performance, and better instantaneous turn at less than about 400 KIAS. You can comfortably loop it from 250 KIAS (280 KIAS is a sensible minimum in ACT in the Hawk). You should only lose in the Alpha Jet when fighting a Hawk 1v1 if you lose tally (have done that!) or inadvertently selected the speedbrakes out (my one Hawk "guns" against an Alpha Jet!). However, the Alpha Jet has a 6g limit (the Hawk is 8g) with a corner speed of approximately 300 KIAS; it is easy to overstress and you always try to drag the fight below 300 KIAS as soon as possible. Also, the Alpha Jet may depart if, at high AoA, you make large, coarse lateral control inputs without rudder co-ordination. If you depart with fuel in the internal wing tanks (i.e. during the first half of the sortie) you will probably cause structural damage as the wings tanks do not have baffles and are an integral part of the wing; you cannot intentionally stall (or spin in FAF aircraft) until the wing tanks are empty. Any departure above 230 KIAS is liable to cause other structural limits to be exceeded also. Compare all of this to the Hawk T1 which is very departure resistant with no concerns regarding speed or fuel weight.
The GAF Alpha Jets do not have an INS. The navigation system uses a doppler radar and the HUD is fed by an AHRS blended with AoA to give climb/dive angles. These are typically in error by 1 degree and may be as much as 2 degrees which, with 1:1 gearing makes the HUD totally unrepresentative of the front line aircraft and is very disorientating in cloud or at night. Most pilots who fly these Alpha Jets ignore the HUD completely. These aircraft also have no ILS and all of the instruments are electro-mechanical. My view is that these Alpha Jets have no real advantages for RAF weapons training over the Hawk T Mk 1A. An aircraft such as the Hawk 128 is what is needed.
I hope that this clears up a few points. And overall, these Alpha Jets are great fun to fly!