Because the value of the British pound has dropped in recent times the UK is now becoming an attractive place for international ISP networks to connect. 

LINX in particular has seen 17  international networks join the peering group out of a total of 19 new members in the last three months.

In order to keep costs down ISPs peer with other networks.  In other words they agree to route each others’ traffic for free.  Running the LINX network obviously costs money and this is funded by the members so it isn’t a “free” service but it does play a major part in keeping down the costs to customers.