Send an email to the CFO of the UK, his PA will then forward it to the high touch customer service team who will seek to find a resolution that does not look like Vodafone are just a bunch of wheel clamping rip off artists.
It might not work, but it did for me with Orange ;-).
The problem is that the billing systems just are not built for Voice and Data. Before you head out the door, if you find that the network is good where you live,work,travel try and negotiate. The SAC for you means that they do have some room for movement but first you have to debate with a higher being.
I particularly like the phrase 'wheel clamping rip off artists' ;-)