It's prohibited by the MATS Part 1, but I couldn't what the rationale is with any certainty. I would think that it's to avoid a possible mis-ident. Remember that I can see much more than my area of responsibility on my radar. There are many service providers with overlapping radar cover and outside controlled airspace more than one unit may be able to provide a service (mil units as well). There is no way of guaranteeing that no-one else has asked a 7000 to ident at the same time, or indeed that a 7000 isn't identing by itself. Unfortunately I can't use judgement or make any assumptions, I must use the radar and pilot reports to get a positive ident and the rules are quite strict. This is taken from the CAP493 (sorry quite long and dull!):
Identification using PSR
One of the following methods is to be employed when PSR is used to identify aircraft. Direction finding equipment should be used to assist the identification provided it has
been approved for such use.
The Turn Method
An aircraft may be identified by ascertaining its heading and, following a period of track observation, correlating the observed movement of a particular Position Indication with:
• the acknowledged execution of an instruction to alter heading by at least 30°;
• one or more changes of heading of at least 30°‚ as instructed by another controller;
• one or more changes of heading of at least 30° reported by the pilot.
A turn for identification does not constitute the provision of a surveillance service. However, controllers should take into consideration, terrain, other surveillance
returns, PSR coverage and the RoA before instructing an aircraft to alter heading.
In using the turn method the controller shall:
• verify that the movements of not more than one Position Indication correspond with those of the aircraft;
• exercise caution particularly when employing this method in areas where changes of aircraft heading are commonly made as a navigational routine.
Departing Aircraft Method
By observing and correlating the Position Indication of a departing aircraft to a known airborne time. Identification is to be achieved within one mile of the end of the runway
unless otherwise authorised by the CAA.
Particular care should be taken to avoid confusion with aircraft overflying the aerodrome, making a missed approach, departing from an adjacent runway or holding
overhead the aerodrome.
Position Report Method
By correlating a particular Position Indication with a report from the pilot that the aircraft is:
a) over an exact reporting point which is displayed on the situation display; or
b) at a particular distance not exceeding 30 miles on a particular radial from a colocated VOR/DME or TACAN (DME). The source facility must be displayed on the situation display; or
c) over a notified visual reference point or prominent geographical feature, in either case approved for the purpose and displayed on the situation display, provided that the flight is operating with visual reference to the surface and at a height of 3000 ft or less above the surface.
The identification must follow a period of track observation sufficient to enable the controller to compare the movement of the Position Indication with the pilot’s reported route. The reported position and level of the aircraft must indicate that it is
within known PSR cover.
This method must be reinforced by an alternative method if there is any doubt about the identification because of:
• the close proximity of other returns; or
• inaccurate reporting from aircraft at high level or some distance from navigational facilities.
A pilot is to be informed as soon as his aircraft has been identified. When operating inside controlled airspace, the pilot of an aircraft need only be so informed if the identification is achieved by the turn method.
SSR – Mode A Identification
When using Mode A to identify aircraft, one of the following methods is to
be employed:
a) Observing the pilot’s compliance with the instruction to select a discrete four digit code;
b) Recognising a validated four digit code previously assigned to an aircraft callsign. When code/callsign conversion procedures are in use and the code/callsign pairing can be confirmed, the callsign displayed in the data block may be used to establish and maintain identity;
c) Observing an IDENT feature when it has been requested. Caution must be exercised when employing this method because simultaneous requests for SPI transmissions within the same area may result in misidentification. Aircraft displaying the conspicuity code 7000 are not to be identified by this method.