the BCN PS2s certainly ended up forward entrance and OPO (photo
here - by j a sanderson on flickr) - not sure if they were built as such or converted. Birmingham's were converted to OPO (although believe they were used crew operated when loaned to Potteries in the early 70s)
Likewise, the Kingston on Hull Regals look like they were (photo here - by regentV207 on flickr) - think K-o-H were among the first large operators to be fully OMO (1972/3?)
Paul Lacey's history of Thames Valley records that the company's last rear entrance Bristol LWL was withdrawn from service at High Wycombe depot in February 1971.
I think North Western kept a small number of Bristol L's after most had been withdrawn - I have a vague recollection (but can't immediately find a reference) about a fairly obscure route somewhere around Buxton where newer / larger buses didn't fit.
Aware (as has been said above) that Crosville had some rear entrance L's in to the 70's but again don't have a date to hand.