More of a "MUST NOT" than a "SHOULD NOT".......
Supplementary Instruction no1 2005 in CAP 493
On being informed that an aircraft is manoeuvring in accordance with a TCAS Resolution Advisory (RA), a controller must not issue control instructions to that aircraft which are contrary to the RA communicated by the flight crew. This includes avoiding action in the horizontal plane, as the controller must not attempt to modify the aircraft’s flight path.