why can't I (or can I) ask for a deconfliction service in VMC and practice my instrument flying without a safety pilot.
Because its contrary to Rule 24
24.—(1) An aircraft shall not carry out an instrument approach practice within the United Kingdom if it is flying in Visual Meteorological Conditions unless the conditions in paragraph (2) are met.
(2) The conditions referred to in paragraph (1) are as follows—
(a) the appropriate air traffic control unit has previously been informed that the flight is to be made for the purpose of instrument approach practice; and
(b) if the flight is not being carried out in simulated instrument flight conditions, a competent observer is carried in such a position in the aircraft that he has an adequate field of vision and can readily communicate with the pilot flying the aircraft.