I am a bit late to this discussion but not sure I agree on jalapeno as sub. The generic green chillis in UK supermarkets tend to be green finger chillis - I have seen milder fleshier ones on sale as 'green chillis' too but they are less common so I'd go with finger chilli if unspecified in a UK recipe.Finger chillis are fairly spicy, certainly hotter than a jalepeno. I checked on a UK supermarket site and it put them at 10,000 schoville units vs 2,500 for a jalepeno.
In the UK it is not common to find a big variety of chillis in anything but specialist stalls/shops or ethnic supermarkets but other common supermarket varieties - birds eye, scotch bonnet etc - are labelled as such.
As bellaCat says you'd use a hotter variety in a Thai recipe, even if type was not specified in the recipe. Birds eye would be a good catch all in the UK for Thai food.