Welcome to afish777, and congratulations on your first post

The traditional easy way out was to take the transit and stay on at T5C, and when you got back to T5A you would be at the border. However, considering the queues and restrictions now at the border I would not recommend this route atm. There is an escorted way out and I would strongly suggest you take that rather than taking the T5 transit to leave. The escort used to be every hour but is now on request, so speak to a BA person once you are airside and want to leave.