Many years ago, I did a familiarisation course with British Airways. Part of the course was a 'jump-seat' ride, in my case EGLL-EGPH-EGLL in G-AWZK. Can't remember the captain's name, but he did mention that he had very little to complain about as regards UK ATC, but he didn't like ATCOs who omitted the words 'flight level'.
So, I took his advice and it seemed to work fairly well. Two digits for FL95 and below, and three digits for levels above.
Headings are always three digits, I added the suffix 'degrees' to all of them, (even those ending in 5), and it seemed to work, well, no complaints anyway.
I didn't realise you were changing from metres to feet this week, so very best of luck.