dera, I am beginning to suspect now that you have a vested interest in US flight schools.
I have done a 2 week PPL in the UK and when full time I completed 12 3 week PPL's in the 13 months I was a full time FI.
And taking a day off and travelling to London to get a visa would mean most people would loose a days work if not 2, they would also have to get to London early morning in peak time which can cost hundreds of pounds or get a hotel which isn't cheap either or fly down again at cost followed by getting from the airport to the centre of London which is again not cheap.
If my suspicions are unfounded I apologise.
I mat add I am a airline Captain these days with no vested interest in any flying school anywhere in the world. With over a thousand hours instructional experience. You won't know the standard of your training until it either bites you in the bum or you take your flying to the next level. As someone that did training in FL I discovered the holes in my training at CPL FI training level thankfully. I then had to sort out the sausage factory pilots on their return to the UK before they were aloud to rent aircraft.