Hi Jeff
Transsec (a unit within the Department for Transport) devised the current hand-luggage rules and simply imposed them on all UK airports and on the airlines using these airports. The fact that nobody in Transsec has ever run an airport or an airline is not seen as an obstacle....
Please see this link:
http://www.dft.gov.uk/stellent/group...ec_613514.hcsp
It seems that you will be OK to carry the cameras in your cabin baggage, but pack them near the top because they have to be removed from the bag and screened separately.
Apart from the size limit for the cabin bag (which is actually quite large) Singapore Airlines told us in October that the single item of cabin baggage must not weigh more than 7kg per passenger. That might be their own rule, though, because the DfT stuff doesn't seem to mention it.
Cheers
Gill