At this time of year, you should have a look at the pond pump systems and clean out the filter. Also, you mentioned a UV filter, in mine, there is a bulb that does this and its always popped by early February so if you have similar then look at changing that, any Maidenhead Aquatics shop will have the bulbs.
Your fish look good and as mentioned when established will look after themselves, I have food as well and chuck a handful in a couple of times a week.
At this time of year, I can't tell from your picture what aquatic plants you might have in there, the pond does look very open so maybe think of a rock formation the fish can hide under to avoid the Heron? I've had my pond cleaned out twice by our local one.
Nice natural looking pond