I'm at goal, but am I too skinny now?

Partly it's a case of people being used to you looking a certain way, and having difficulty reconciling your new appearance with their old idea of you. Partly it's that irritating way that our bodies tend to lose fat from around the face much more readily than around the belly, so while you can look really lean with your shirt on, there's still a bit of fat to lose. And yeah, partly it's a body comp issue from lack of resistance training.

In your situation, as you've done next to no resistance training you will see a reasonable amount of muscle gain/continued fat loss even if you eat around maintenance, as long as you choose a decent training program that really stimulates your muscles (the oft-mentioned newbie gains). I'd suggest finding a decent program (something like stronglifts 5 x 5 if you want to keep it simple) and following it for a few months while eating around maintenance, or a very slight deficit.

But still, you've done really well so far and even if with your shirt off I doubt anyone would think you look unhealthy, either emaciated or fat, just a bit soft/unathletic.

Gyms here are super expensive, I was thinking maybe some ~3 per week body weight exercises roughly based on a "nerd fitness" site I saw (I think it was 15 push ups, 10 squats, 10 lunges, 10 lifts of something heavyish like a milk jug with each arm, 10+ seconds plank, 20 star jumps, repeat the whole set another 2 times - doesn't take very long at all, I was pleasantly surprised) if that would be any good. But I'm sure I can get a gym membership going if need be.

Great, thanks for the additional advice, all helps. I think the general consensus is that I could definitely use a bit of improvement to composition/muscle and I don't think that's a major problem or would need to be expensive or overly time consuming. But despite this there isn't any recoiling in horror (though I'm sure there'll be one eventually!), which is a good sign, and I haven't overshot and gone too thin.

I used to dread any talk of gyms or other organised exercise as I had the same vision that I think many others do - that you have to spend every free waking hour trotting along on a treadmill bored out of your mind or spend hours shifting weights (instead of minutes). Or that you have to start playing around with protein supplements. But it seems to me, correct me if I'm wrong, that unless you have certain ambitious / competitive goals you don't need to spend your life on it to see some great benefits, or do anything too special to an already decent diet. In fact to get here, I've been getting away with a pretty traditional balanced diet (which my body is quite happy with compared to before when the nearest it saw to a vegetable in an entire week was the bit of shredded cabbage in a doner kebab on a Saturday night), and not the low carb / high protein paleo etc that many recommend (I love rice and pasta).
It is perhaps going to be more important now with the muscle stuff, but then I'm not looking to "get ripped in 6 weeks" but quite happy with small, gradual, sustainable long term improvements.

Compared to the effort of losing a third of my own body weight, a 15 minute strength workout a few times a week really doesn't sound too difficult, even if it takes just as long (a year) to see clear results.

Currently I try to go for a 20-30 minute brisk walk every lunchtime which I am fine with, it gets me into town for a look around. A lot of the time I cycle to work and back now, it's not far (downhill there, 7-8 mins, uphill back 12 mins) but with these extra activities even though they're short and sweet I think they add up and also just the overall "more active" mindset helps.
Some other times of the year will be more tricky (Northern UK. We get a lot of rain), maybe that's where the gym will come in, will have to see what happens.

Fortunately I do really enjoy at least one physical activity: mountaineering.. which is one major reason I wanted to get my body under control. That's fairly seasonal again in that it's more of a spring/autumn thing (too hot in summer, and many of the fells can be quite dangerous in the icy winter) but something else that helps. I'm hoping to be getting into it for regular weekends again within the next few weeks as summer tails off.