Well, that looks like a really nice menu. My sister lives not far from Stratford, so we might try that place some time. Do let us know if you liked it!
Actually, I think you have more choice there than you might think at first glance.
For a starter I think I might choose the King Prawns and salad and not have the mayonnaise.
Or possibly the roasted squash salad - again, ask them to leave off any dressing.
I would definitely give the deli boards a miss, unless you could share one with somebody.
For the main course I would have the sea bass - I know it says pan-fried, but it is a reasonably firm fish and won't absorb too much fat. Or the steak, leaving out the chips and only having a little bit of sauce.
Some of the things in the starter menu can also be served as main courses, so you could have the squash salad at that point if you liked, instead.
Your best bet as a pudding would be a lemon or mango sorbet - it will have sugar in it, but no fat. (Definitely not the cheese and biscuits - there is always far too much and it usually looks to me like tomorrow's lunch rather than a dessert!)
(Mind you, if it were me, I don't think I would be able to resist the flapjack and poached pear!!!!)
I think it is such a great idea that restaurants publish their menus on their websites, so we can have a proper think about it in advance.
The most important thing of course is that you have a good time!