Get it done in the UK. You have to book and spend a day at it, but it is same day. Been there, done that. Newport is full of people hanging around waiting for passports!
Thanks for all the helpful and reassuring replies!
A few years ago I entered the UK on my British passport which had been renewed in Madrid (due to my place of residence being in the Canary Islands at the time).
The immigration officer took it and looked at it long and hard, then asked me to step out of the queue. Another guy comes along and does another deep inspection. Then says to me that the photo has been put into the passport at an angle, i.e. not straight as he said. I just waited while he inspected every page (still just out of the queue). He then said and I'll quote as I will never forget it - "Next time make sure you put your photo straight." I said so you're asking me to make sure that next time I should make sure I put my photo on straight in my passport? Yes he replied. I then asked for a complaint form and was told they had run out. Me being me I said well that doesn't surprise me. Blink blink from him. And then I passed through.
OK - a happier story.
Once upon a time (a long long time ago) I arrived back at Heathrow after an overnight flight and could not find my (UK) passport. After rummaging around in my handbag for some time, the office said "you're clearly British, just go through". So I did. And found it in a side pocket of the bag when I got home. I wouldn't worry if I were you.