Every UK mobile network has committed to switching off their 3G network, some sooner than later.
Here what to expect:
EE: https://ee.co.uk/help/mobile/manage-use/3g-plan-changes
O2: https://www.o2.co.uk/help/network-coverage-and-international/3g-switch-off
Three: https://www.three.co.uk/support/network-and-coverage/our-plans-to-switch-off-3g
Vodaphone: https://www.vodafone.co.uk/help-and-information/3g-switch-off