Relevant regulations include:
Art 126 (1) An aircraft to which this paragraph applies [PT and instruction] shall not take off or land at a place in the United Kingdom other than:
(a) an aerodrome licensed under this Order for the take-off and landing of such aircraft; ...
and in accordance with any conditions subject to which the aerodrome may have
been licensed or notified....
Rule 39 Visual signals
39.—(1) Subject to paragraph (2), the commander of a flying machine on, or in the pattern of traffic flying at, an aerodrome shall—
(a) observe such visual signals as may be displayed at or directed to him from the aerodrome by the authority of the person in charge of the aerodrome; and
(b) obey any instructions which may be given to him by means of such signals.
(2) The commander of a flying machine shall not be required to obey such signals if it is inadvisable to do so in the interests of safety.
Rule 40 Movement of aircraft on aerodromes
An aircraft shall not taxi on the apron or the manoeuvring area of an aerodrome without the permission of either—
(a) the person in charge of the aerodrome; or
(b) the air traffic control unit or aerodrome flight information service unit notified as being on watch at the aerodrome.
My conclusion would be that the use of such undershoot for flights not requiring a licensed aerodrome would be subject to the permission of the aerodrome owner, but not, in itself, illegal.