I have a friend who was refused clearance to route via SAM despite being IFR on a flight plan. He assures me this is not allowed, and he should know. It also suggests that some of the advice here would not be enough. A flight plan warns them you're coming, but doesn't give them extra controllers (I am informed, fairly reliably, that this is a problem at Solent).
Had he been a recently-qualified IMC pilot rather than an IMC instructor (nothing in the flight plan indicated his qualification, the type was a light single on the registration not a callsign) he might have had some difficulty routing in IMC unexpectedly around what is complicated airspace, with the proximity of Odiham and Wallop plus Bournemouth, danger areas and some HIRTAs and the lack of RNAV on the aircraft.
Having said that I have never had a problem with Solent, and was welcomed in as a professional GA pilot when I went to EGHI.