Brendan, you're embarking on a rough road, no question. But you'll get through it.
I know what you mean about the numb tongue. Even though I only had a neck dissection they must've nicked some nerves because the left half of my tongue has remained numb since then - 2 1/2 years ago. Glad your swelling has gone down and feeling is returning. When it comes to nerves, as I've found, there's no way to determine how much or little will come back. My RO has told me that she's seen a return of nerve activity up to 5 years post-treatment.
My advice to you (others will weigh in similarly, I'm sure) is to try and eat as much as you can now before the treatment begins. I know it can be tough, but it's necessary. If you can't swallow enough to put on weight try liquid nutrition. Dunno what's available in England but I'm sure you have lots of options. I lost 30 pounds during radiation and I didn't have chemo.
It might be wise to get a thyroid test now so you have a baseline level. Radiation can muck with it and many of us, myself included, are now on lifelong thyroid medication as a result. I'm sure your docs have told you to visit a dentist as well for tooth evaluation.
You'll have many more questions and people here will answer them. I must admit I was a bit surprised by your 60% figure... maybe some doctors do throw these things around but I never believe them. Statistics are statistics; people are people. I never for a moment believed I wouldn't get through it all. You sound like a strong guy so remember: you're unique and not a statistic!
Have you been tested for
HPV, btw?
Courage, and keep us informed along the way.