I am going to go a little against the grain here.
Your DS is now living almost full time (during the school year anyway) with his peers.
As a boarder, social life is extremely important. He lives with these kids. He handles himself in school. I can see how he would feel impatient with your concerns.
My approach would be to help him plan his route, find the exact house, and how to get there from the Tube stop. He is 14. You can look at that fact and say 'He is still a kid', or 'Time to start learning'.
I think he is just about at the right age to start navigating London and public transport in general. This is not the only party invitation from a London friend that will come his way this summer or over the next set of holidays. I think you need to take a deep breath and facilitate his first foray.
If not now, when would you see him beginning to learn the ins and outs of train travel and finding an address? Help him by figuring out the route.
Wrt the lack of a phone number for the parents - forget that avenue of keeping tabs. More and more from now on you will encounter parents who allow their children to run their own parties. It would be nice to have a phone number and an email address, but there is no guarantee that you would get any response from a parent for the duration of the party.
Instead, you need to work on establishing trust between your DS and yourself. Approach this party with a positive mindset. Show your DS that you are confident and not out of your comfort zone.
Ask that your DS contact you at every step along his route.
Require that DS reveal the address of the party and name/s of the parents and make an honest effort to get a phone number.
Important:
Insist that DS text you back if you text him, and set a few times when you will text and expect a response within a few minutes - 21:30, 00:30, 03:30-04:30.
I have found that 00:30 is a good time for communication - it is when parties take on a direction that might not be to your liking or to your DS's liking.
Above all, you and DS need to come to a firm agreement that he can get in touch with you, or respond to your text at a set time, with a request to be taken home, and that you will not ask any questions about what happened. He may ask to be picked up from a location a little away form the party address. You could agree that all he has to do is text 'X' or some other letter, or a number, etc. if he is leaving the party for any reason.
His phone should be set with volume on to facilitate communication.
Any lack of responsiveness (no acknowledgement of texts) on his part to be met with consequences either during the following week or whenever another invitation materialises.