could i use my own aircraft to do IR training over here in the uk?
Another big subject. ALL ICAO training, done in any aircraft, any reg aircraft, is allowable towards any FAA license/rating. The catch is the checkride, for which the European options are seriously limited. This is why one may as well head for the USA.
But if one is anyway receiving training here, may as well do it in such a way that it counts towards the FAA stuff. For example do a 250nm airways flight in the IMCR training, will also count as the airways flight in the FAA IR. And structuring the Night Qualification training towards the FAA PPL too, etc.
Its a piper warrior PA28-161 FM imune?
It depends on the equipment fitted.
As regards twin flying, twins cost at least 2x more to operate than singles and in general do not go any quicker. IR training in a twin costs a lot more. It all depends on one's budget but I think the days of piston twins are over - certainly avgas ones.