Just looking at the UK....
If you are looking purely at G- reg helicopters with current a/w ticket, then I think Hiller G-ASAZ (1961 build) is the oldest - this was the one owned by Hields Aviation which failed to reach its reserve in a recent auction of Bond memorabilia
For Bell 47s, the honour is with G-LHCI up in Scotland, 1961 build - others may recognise this as G-SOLH or its original reg G-AZMB
If you widen it to foreign-registered UK-based, then
hillerman was right to point us to the N registered Hillers, but while the one which appears to live near his location (
that's how you know about it...) is "only" a 1956 build, while a Norfolk based one reg N831M looks to be a 1952 production. But is N831M airworthy?