Laurence - could I suggest, possibly going out into the countryside somewhere remote and doing some photography? You shouldn't
have to, but if you do then you are possibly likely to get some peace and quiet be able to shake off the worry that you are going to be accosted by some lowlife little hitler, and thus be able to actually enjoy it, which may reignite your passion for it.
I'm guessing there's specific technical ornithological reasons why you were taking photos at the thames, such as the grebe being a bird of particular interest that nests there. If you got away from the city for a bit you might not have that specific interest, you might possibly even have to branch out into other types of bird or just nature in general - wild creatures etc, but most importantly it would enable you to do what you enjoy doing and get back into the spirit of it.
When you've done that, and enjoyed the lift that it WILL give you, THEN you can think about a pragmatic approach to the possibility that you could get accosted again.
I suggest that by being prepared for such a possible occurrence in the future, it won't depress you as much, and that means understand why they are doing what they are doing, what they can do, and what you can do. Why: because it's their job, why is it their job : because the job's there and they're individuals with not too much between the ears but a power hungry ego - don't think more about it than that. What they can do: not much really, apart from have the right to be treated with civility. What you can do: you are legally allowed to take pictures, you also have the right to be treated with civility by them. You also have the right not to be arrested if you haven't committed any crime. (I think.) Familiarising yourself with the law in this area might make you more comfortable if (when) you do photography in london in the future, and quoting it back at any little hitler that accosts you might make them back off.
I agree that the world has gone a bit mad though. I had a bit of a worrying moment the other day: I'd bought some new forks for my bike and I had them in my van, so I put them inside the sleeve of my jacket instead of loose in the back of my van so they wouldn't get scratched. When I got home I wanted to take my jacket and the forks in with me so I just grabbed my jacket with the forks still in it....fortunately i'd only took a few steps out of the car park when I thought WOOAAAHH!...shiiiiiiiittt !!! i know what this could look like...