It depends......
Yes, you could argue that they are a development of heather floors or even of the mesh ventilation insert present in the floors of the Irish CDB (Congestion District Board) hives which have been on the go since at least the mid 19th Century and since Northern Ireland is a part of the UK, you could thus argue that OMFs have always been in the UK!!! As a specific element within integrated pest management they are a more recent development and I'd be inclined to agree with Finman's suggested date of 1987. Alternatively, don't the Yanks refer to them as Pettit Boards? When were they first developed and could someone have brought the idea to the UK?