My advice is to lose some weight. Not being funny here, but seriously, if your weight is an issue anyway, maybe this is the motivation you need.
It's not too difficult to lose a stone or so over about 2 months. Eat small sensible meals, often throughout the day. Keep off the drink. No take aways (I know I know, not easy) and exercise. By exercise you only need to raise your heart rate and 20 minutes, 3 or 4 times a week on a running machine at 6 mph will be enough for that. It's not as if you'll need a huge lifestyle change and it'll do you good too. Good for keeping that Class 1 current.
You're right that the more you weigh the less your student can weigh. There are a couple of instructors I know flying R22's who are definitely over 17st. They are breaking the rules but hey, nothing I can do about it.
There are more R22's available to teach on in the UK than the other types, but that's not to say you won't fly other types. I take it you're in UK.
Good luck.