If I recall correctly the UK had a derogation until April next year from the requirement for EASA aircraft to have an ELT fitted. After that time if there is no further derogation then you will have to have an ELT.
Also other states (such as Holland) do not have derogations so legally even in a UK registered aircraft you should have one when flying in those countries. Whilst PLBs are a good idea (and probably better since ELTs don't work underwater in the case of a ditching) they do not fulfil any legal requirement for an ELT.