|Image credit: US Navy.|
Currently flying around the UK's airspace (according to the folks on Fighter Control ) is a US E-6B Mercury, or TACAMO. This beasty is a platform to send EAMs (Emergency Action Messages) to deployed submarines and other assets around the world, in the event of 'the big blue flash'.
To quote Wikipedia (I know, not the best source):
It does this by maintaining the ability to communicate on virtually every radio frequency band from very low frequency (VLF) up through super high frequency (SHF) using a variety of modulations, encryptions and networks. This airborne communications capability largely replaced the land based Extremely Low Frequencies (ELF) broadcast sites that became vulnerable to nuclear strike.
What this means is it's stuffed full of exciting comms gear (as well as the mission specialists to use it all) and can basically transmit anywhere, any time with as much power as possible to get the job done.
They also have a 26,000ft long trailing wire antenna for VLF communications with Nuclear submarines - some very old technology - and alongside this they also use VOIP (Voice over IP) to communicate to presumably 'the folks back home'. Very sexy bit of kit.
There are plenty of frequencies out there on Google that the TACAMO bird has used in the past, but most of your scanners/radios will have a search button (230-400MHz is a good start) so get searching! As well as UHF, get searching on HF - although my fellow monitors have had no real luck finding any downlinks or comms from the E-6 aside from air traffic control.
Maybe you'll have more luck - let me know.