Skipness - I certainly believe so (but am hoping to be proved wrong).
Everyone believed to be arriving from ROI at LGW South is put through a single channel (manned by a single airport employee) in the International Arrivals Hall with only a boarding card stub as proof of identity and of origin. Possibility (very remote in my experience) that Police may be on hand to ask for further identity checks...
Of course, official answer is that all pax originating from ROI will have cleared EU Immigration check there (or wherever their original point of entry to EU was). My point is regardless of that assumption, you can still circumnavigate UK Border Control at LGW South by the method I have described above.