Note: With the UK's exit from the EU ("Brexit"), it is unclear whether all providers will continue to offer EU roaming benefits (voluntarily). In case of doubt, this should be clarified again directly or - for example when traveling through several European countries - buying a SIM card on site should be planned when entering the UK.
In the UK, there are 4 network operators: EE, Vodafone, O2 and 3 (Three). These use the frequency bands common in Europe.
Prepaid is most commonly referred to as "pay as you go (PAYG)" in the UK.
With all Internet access in the UK, only adult content is displayed unless this is explicitly deactivated (with proof of age). However, quite common services often fall into this filter, which is why it is advisable to check the age at the time of purchase.