Immigration checks and security checks are quite different. In the UK there is no physical passport check when leaving the country. It is electronic and done via API and boarding pass scanning. Going through security when leaving the UK will take nothing like as long as your wait for immigration coming in. A 0530 pick-up will give you plenty of time for a 0830 boarding time. I would personally go for a 0630 taxi, unless you want lots of time in a lounge. Others would go even later than me.