Official procedure from my side (Post Office):
Officially no longer legal tender from July 1st.
Notes still accepted by retailers, but not given back out, must be banked into takings - so check your change when you go shopping!
You should refuse any attempt by any retailer/bank/whoever to give you one of the outmoded notes.
Bank of England will always exchange any outmoded notes, regardless of how old.
We haven't been given an official date to stop accepting them either, usually it's about 6 months or so, so I'd expect it by the end of the year.