Govt/MAFF's problem is to try and find an exit strategy to regain disease free status at least cost. The big question/challenge they face is how few animals/flocks/premises can they get away with testing and satisfy the EU and other countries that UK id disease-free.
Fact is no-one has a clue how widespread FMDV is in the UK - it could be just about anywhere - esp. within sheep - OTOH it may not be.
The reason they would only seek to test on farms with both sheep and cattle is that the vast majority of cases so far have been on farms which have both sheep and cattle - so they'd expect it to be more likely to be found on such holdings.
What they are keen to avoid is a demand to survey sheep flocks nation wide - which might yet be the demand from the EU and other countries - before regaining disease-free status. Why? Cos they know that would be in excess of 10,000 farms and require testing of well in excess of 1 million samples - the labs now have capacity to process 60,000 samples a week working flat out round the clock - so it would take about 6 months (if I got the sums right) just to process the samples - then consider the logistics and the costs!
Personally I think it's wishful thinking on Govt/MAFF/NFU part to believe that export restrictions will be lifted without more widespread testing than they hope to get away with. And lifting of restrictions is a far cry from reestablishing export trade anyway - one only has to think of BSE. Gareth Davies, former head of CVL Epidemiology Unit and member of the Chief Scientist's FMD Science Group, said may weeks ago that he thought it would be 3/5 years at best before this might happen.