I had dinner at Mangetout in Southend last night. It's the first time I've been, but I'd heard good things about them.
They are quite hard to get booked into at short notice, but book in advance and you'll be fine. The easiest way to book is online as I'm yet to get through on the phone.
Of course they do lots of off plan meals (burgers, ribs etc) but I chose on plan and I didn't feel deprived at all.
I went for the Mangetout salad, with steak for my protein. I added on grilled halloumi to use as an A (it's sightly more than 1 A portion but I won't fuss about that in a restaurant). Aside from allowing for perhaps some oil on the grill, it's very healthy and very fuss free for eating on plan.
There is nothing like being able to make a healthy choice without making a show of yourself!