Most likely some combination of:

    1. A chunk of the customer base (financial sector, hyperscalers etc.) that wants the low latency and who are price insensitive because of their deep pockets.
    2. If peering matters to you, then you're limited to where the IXP is, which is usually only at the major London sites.  LINX, for example, have LINX Wales, but that is not interconnected with LINX London, so you either need to get space in London or pay for fibre capacity back to London.
    3. Fibre coverage outside large conurbations in the UK has traditionally been shit and to varying degrees still is.
    4. The rural areas don't have substations ready-to-go and the NIMBY's come running if you propose building one or anything else in their backyards (see protests about new wind farms).

There are various locations outside of Central London but within the M25 boundary. But YMMV when it comes to being any less expensive. I suspect you will find the Outer London market has "hardened" over the last few years.

Verging into cynical territory, marketing might come into it a little bit. "Telehouse London" sounds cooler in the customer presentation "Telehouse near some village you've never heard of".

So datacenters build in London as the connection/electricity price is same as building in rural areas and they'd obviously prefer being closer to users in London.

Here's one example map: https://www.colo-x.com/data-centre-database-map/