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Not all 07xxx numbers are mobile numbers
I'd forgotten about pagers (even though I had one back in the 90s) and those stupid 070 'personal' numbers. That leaves a bit less than 8×10⁸ issuable mobile phone numbers, which is only about 10 per person given that population is likely to hit 7×10⁷. It's pretty tight if much of the internet of things is going to be connected over the mobile network.
Not all 07xxx numbers are mobile numbers, only 071 -075, 077 - 079, 076 are pager numbers (Try buying one now though).
Also 070 are personal numbers, and are charged differently to mobiles.
Also, a tiny number of them (07700 900xxx) numbers don't exist, and are for use in films etc, like 555 numbers in the US.