This could cause problems with your BA booking when trying to get back into T5 as BA would need to reverse your original check-in to clear the existing ready to fly marker and then check you back in with no guarentee you'd get the same seat(s) back. Not a block on you doing this but something to be aware of..

There is a way to get to T3 from T5 when originating at T5 but it involves getting on the intra T5 transit to T5C and then staying on when it returns to T5A to be able to enter the arrivals and then the flight connections streams. More details on doing this are on the BA Board.

I don't think transit security at either terminals is a major issue these days so it's down to your personal preferance on whther to do this or not. People have all sorts of reasons to visit another terminal and remain airside - such as meeting relatives on other flights - and people doing that sort of thing aren't clogging up the lanes.