Well, a lot of it will depend on where you're flying to and at what time of day, and crucially your visa status.

I'm assuming from "early AM" that you'll be off the inbound by 0800 UK time, with an onward departure of 1600ish later that day... You will need to be back at LHR probably around 1430... if it's busy, 1500 will be cutting it really fine.

It takes about an hour by Tube from central London to T5. So you would need to be heading back by, say, 1330...

Let's assume your meeting finishes at 0915-0930 so you can be in the city by around 1015-1030. That gives you about 3 hours clear.

For me, unless I was seized with a burning passion to see Buckingham Palace in the flesh, that would be too short to make it a worthwhile trip. YMMV

All of the above assumes no horrendous queues at the Border or T5 Security, neither of which is a given by any means :

(However, before considering all that, you will need to have the appropriate visa to enter the UK. Without it, it's 8 hours in the lounge and shopping mall...