Matters are further complicated at LBA by the fact that there is no GMC. To digress a little, the amount of traffic during daylight certainly warrants GMC but the present shortage of taxiways makes it impractical. Therefore TWR handles all aerodrome traffic (except when the Delivery position is open) with the result that a significant amount of RT is handled by a single controller. The likelihood of ground vehicles and a/c transmitting simultaneously means that messages may require repeating, partly or in full. Not an ideal situation for a busy ATC position.
On the other hand, certain Aerodrome Authority vehicles (e.g. Checker, Tels, Fire) are granted "free-ranging" status on certain parts of the manouvering area, which I guess is similar to "own look-out" at MAN. This must help a little.
As for Bournemouth, I wouldn't be surprised if the TX/RX switch was similarly configured to that of LBA hence the same situation described at that airport. I don't think transmitter power would be significant: most handhelds give at least 4W and vehicle mobiles more. In any case, there would be a UHF to VHF repeater somewhere on the airfield to provide complete coverage.
I maintain that it doesn't make sense to have only a "semi-operative" cross-coupling facility which seems to be the consensus on this thread so far.
MC