Then existing 07x numbers would be made 071x numbers, and repeat.

But the existing 07x range is effectively 9 digits long, (with a few chunks out of it) so we're probably good for a while.

We've not started 072 and barely touched 076 (and 071 is currently unused but that's sensible if the existing might ever be bumped to start 071).

Our population count is a good way way away from being 9 digits.

Already (because of number porting) you have numbers from networks now home in the middle of number ranges of their competitors.

But I think we're more likely to see a separation of number/device before that 071x happens. Devices or SIMs will have an IPv6 address and your number is separate but accessible. Time will tell.

What does it matter if the numbers are contiguous? Just keep allocating space for mobile numbers.

Surely you can just add more digits? Once 07XXX XXX XXX is exhausted, you could start issuing 071XXX XXX XXX numbers. It's been done with POTS numbers before (e.g. Sheffield - transitioned to 6-digits in 1965, 7-digits in 1995)

https://en.wikipedia.org/wiki/0114