In the case of Gatwick I would ask for an IFR transit "not above 2500ft" (if I had only an IMCR). They should get the message from that.
They'll get the message that for some reason you don't want to go higher but they won't get the subtlety that it's because you've got an IMC rating as opposed to an IR. Very few controllers fly these days and know that the two qualifications exist. Don't be surprised if they still ask you if you could accept a higher level as higher, for them, may be better.
For IFR you'll also get a maintain level as opposed to a not above level. That maintain level is 99.9% certain to be a round number, therefore Gatwick's case 2,000ft (assuming it's terrain safe, haven't checked). The reason it probably won't be anything other than a round number is because all the other IFR stuff will be flying around at round number levels for the most part i.e. 3,000ft, 4,000ft etc and vertical separation of 1,000ft is required.
Notwithstanding all of the above and despite what everyone here seems to think, the low level CAS surrounding airfields isn't that big when it's filled with relatively fast moving and large IFR traffic so fitting in slow moving ad-hoc IFR at low levels isn't necessarily straightforward, especially when as soon as you cross the boundary into the CAS under IFR 1,000ft and/or 3nm separation is required at all times.
In my humble opinion, assuming the weather is suitable, for low level ad-hoc transits VFR is by far the best option unless it's quite quiet with other IFR traffic at the time.