Quick AAIB scan shows a few midairs, and this recommendation:
It is recommended that the Civil Aviation Authority should initiate further studies into ways of improving the conspicuity of gliders and light aircraft, to include visual and electronic surveillance means, and require the adoption of measures that are likely to be cost-effective in improving conspicuity.
I think the last point is lilely to be the problem ("cost-effective") given caveats such as
Accordingly, it is considered that the organisation best placed to lead any study would be the CAA.
However, it would be necessary to include other organisations, such as the BGA, in any study.
FA - of the 4 midairs I found they involved 4 gliders, 2 helicopters, 1 microlight and 1 "light aircraft" (I think). So I am not sure an additional height / speed band for, say, Xpdrs, seems warranted by experience?
To further the debate, maybe worth someone doing a search and seeing how far the CAA have got (rec was 2005)?
NoD