I equate 100NL live to 5 or 10NL online with lots of bad players limping and calling isolation raises with a majority of their range just wanting to see a flop; moreover, being non-thinking poker players they'll donk out and play their hand disregarding that you raised pre-flop and could have a strong hand.

With that said, MP could be donking out with a pair of 7's, such as A7 or 4x for trips. Agreed since two of the 4's are gone, the combinations of 4x hands he could have is reduced by 50%, but there are still 24 combos of A4, 54, and 64, which he might call with pre-flop. He could also donk out with 16 combos of 65, 12 combos of A7, and maybe some weaker 7x combos. He might even donk out with 88 - 99 like you said, adding 12 more combos.

Since you are ~ a 2:1 favorite with combos that you beat versus combos that beat you, let him continue to value-own himself. If you raise, he will probably fold 7x, 88 or 99 and only continue with trips or a boat. If you flat, BB might also come along with a weak holding that you beat and UTG might donk bet out again OTT.

So I'd probably flat OTF and if a bad card doesn't hit the turn look to get it in.

The amount of combos you've said then the odds against them is something I need to learn. Do you have a video or article on this please?

I did make a mistake in this hand later on but so far had the same thinking and flat called. Then the BB raised it to £25 and utg folded. Where do we stand now? Looking back maybe I shoul have got it in here?

Depends on how UTG reacts to a flop raise. Does he continue bluffing on later streets so just call. Is he overplaying hands like 88-TT often and stacks off light? Then get it in. BB should not have too many 4s in his range.