There are a number of UK operators who will carry out survey work in piston twins carrying third party kit. There are modifications that can allow fitting to some aircraft types without requiring an STC. However this only applies in limited cases.
You're probably better to speak to these companies directly.
RVL Group
2Excel
APEM Aviation
Ravenair
Directflight
Diamond Executive
These are the companies I can think of, off the top of my head. I'm sure there are at least a couple more to throw into the mix.