Mine still pases emission test with removed precats and main cat replaced by 200cpi racing one. And in the period when I had no precats and still had the OEM main cat it passed the test with big reserve. I think keeping precats and removing the main one will be OK as well.
I would advise against it, removing the pre-cats is fine, remove the main cat and you'll quite likely fail an MOT. The pre-cats are there only to work when the engine is cold, they are a lot smaller and closer to the engine so the warm up quickly, but they will not do anywhere near as much as the main cat does once warm. If your going to do it I would recommend you take it to a garage and get them to test the emissions pre-main cat.