We normally have Large Ranunculus in the Upper Thames area, but it is not unknown for the Feathered Ranunculus to wander quite a lot further inland from its normal Southern coastal areas.
This happened on the night of 11 October when one turned up at the trap run by Richard Ellis at Chorleywood (just in Bucks). The identity has been confirmed.
It is usually a bit smaller and duller than Large Ranunculus, but in males the antennae (apart from the final 20%) are obviously feathered. There has been a previous Bucks record, 1992 at Lavendon right at the Northern tip.
I don't know about Berks or Oxon.
If you spot one your county recorder will want conclusive proof, so make sure any pictures (of a male) shows most of the feathering of the antennae, also save the specimen and let him know immediately.