I think I'm relaxing about this...

The trip to the UK proved interesting. H didn't talk to anyone (except once to my Mum and I think it was by accident, him thinking she was me), BUT he non-verbally communicated quite a bit with friends and family, none of whom he'd seen since he was 1 and 3/4, so unlikely he'd remember them (i.e they were strangers).
He was happy to talk in English to me or hub or siblings in front of people, and he must have realised they'd be understanding him. This has been the prob at nursery that he'd regressed to not even speaking English in front of anyone (including children) there. Maybe the overall stress levels were down as everything was in his first language?
Other points of note, he'll talk to his Uncle in Pakistan, on the phone, who he's not seen since he was a few weeks old so is completely unknown to him and still talks to his grandmother there on the phone too. This seems strange as the phone is usually more stressful for anyone (I remember people who stammer find this particularly hard) and he does use the odd Urdu word which is his 3rd language really (only a smattering though). I can understand his willingness to talk to her as she's lived with us for several months at a time, on and off since he was born, but his Uncle? and men did seem to cause a greater anxiety stranger reaction when he was a baby.
Also the other day, something I didn't personally witness, but my Swedish friend and my kids reported that he was sitting with the friends young daughter in Biryani's cot, looking at books together, and he was telling her the names of some of the pictures in Swedish (plus according to M I think, rude Swedish words ?!).
Now I'm inclined to the 'less is more' attitude in terms of intervention, and pray Allah through the course time increases his inner calm in difficult situations to let the words come out.Ameen.