I don't often knock Prestwick, but feel inclined to point out that when NATS had the contract there was (omitting the UHF, which doesn't apply in this case);
118.15 TWR
119.45 RAD/VFR's depending on what year!
120.55 APP/RAD
121.8 GND (yes!), providing at least one spare (and maybe two spares, if selectable) for this particular occurance.
Edited to give correct freqs!
[ 06 November 2001: Message edited by: Bigears ]