I visited the Covent Garden store and demonstrated my long-standing problem (since day 1) to a Genius Bar representative. I am now on my fourth iPhone X, which suffers from the same GPS inaccuracy as the ones that came before it (first two were replaced by EE, and the third time was replaced by Apple in December);
Over the first month and a half I was in touch with Apple's engineering team through a seniour support person. I was not allowed to interact with the engineers directly and had to wait for weeks only to receive a two sentence reply, which I found insulting. Especially after going through lots of testing and submitting videos of the problem.
All of the phones exhibit the same problem with highly inaccurate GPS. The problem was confirmed in store but the replacement device I was issued had the same problem, which was discovered as I requested to test it out on the spot. As such, an official store to engineering case has been raised and escalated- it seems as if this is software, rather than hardware related according to an engineer that was there on the day to provide training.
I was advised to try and get another SIM card replacement from EE. After a few days of testing, I can also confirm that there is no change in performance as GPS is just as inaccurate as before. At the same time, my corporate phone and tablet on the latest iOS version (6s&iPad Pro 10.5) have no issues whatsoever.
To make things even more ridiculous, my flatmate's new iPhone X from the same carrier is not exhibiting any of the issues that I have been dealing with since getting the phone in the first place. Go figure!