I would practice on your British accent.

Watch the movie Planes, Trains, and Automobiles for research on your modes of transportation.

Bring extra floss and toothbrushes to hand out to the indigenous people.

Ask if there are any real restaurants that serve actual food such as Italian or French.

Tell them that German beer is the best.

Avoid anyone from northern England or Scotland because you won't be able to understand them.  Perhaps a southern can be your interpreter if you do encounter a northern.

I don't know anything about England, other than how it's supposed to be good fun if you insult the local soccer team, but I can say that going on trips by yourself can be both lonely and enjoyable. That's okay, though, really, because going on trips with a friend can be both annoying and enjoyable.

The positive is that you can decide where you want to go at any point during the day, including just wandering around, and you get to eat where you want. That's a pretty darn good positive, honestly.

It's also a good self-confidence builder. I mean, you're never going to see most of the people you mean again in your entire life, so what if you screw up? Be calm and assertive because there really is no need to worry about perception. Pretend you're James Bond and you're on a mission.

I'm not going to lie to you.  Going on a trip by yourself can be kind of lonely at times, depending on your mood.  Sitting by yourself in the airport, for example, or at a nice restaurant.  I've learned to go with take out/delivery back at the hotel because being the only one sitting by yourself for a meal is no fun.  I did once, at a nice hotel restaurant, have another single person ask to join me.  It was kind of awkward but also kind of interesting.

It just completely depends on your mood and outlook.

You can mitigate part of that with immersive activities.  Going to places where people are clustered in groups talking can be bad and you'll feel left out.  Going to immersive places where you don't need to talk is usually good.  Going to see Cirque du Soleil by myself on a trip to Las Vegas was excellent, for example, and I enjoyed going by myself to the Aquarium in Boston on another solo trip.  If what you're looking at interests you, then it doesn't matter if you're with someone.

Geez I'm not much of an expert regarding UK, but you can't do much wrong with London and Edinburgh (even if I haven't been to Scotland myself).
York is pretty neat as well.

Have only been to London for a day, so I only did the standard tourist program. Take a look at Tripadvisor for some popular sightseeing stuff.

During my Iberian Peninsula trip this year I slept in various hostels and was very satisfied. As long as the whole point of your trip isn't to calm down, but to see new things in various cities (which is obviously a cause for some stress) it's definitely good enough.

Aside from RyanAir, EasyJet is also very popular in the UK.

Personally, I buy everything I can beforehand, so I don't have too much hassle with money or printing tickets and whatever else could go wrong. I'm also pretty stingy when it comes to food. Most of the time I only go to local supermarkets and buy baked goods. Something that is cheap, fills me but is also easy to transport.

EDITED my post.

If you can get more time in London, and you're interesting in that sort of thing, I always recommend both the Imperial War Museum and the Tower of London. Both a run on a donation basis, so if you're tight on money you can ponse right on in.

In Edinburgh, I recommend Edinburgh Castle and an underground tour. There's quite a few of 'em around, and I can't remember which one I did, but it was pretty cool. Essentially, the Edinburgh of today is actually built on top of the Edinburgh of hundreds of years ago.

In terms of being socially anxious.... a socially anxious north American is a rockstar by Brit standards. We're a nation of social tards. I'm obviously exaggerating a bit, but culturally, Britain is a much more timid place. Places are less likely to put you on the spot than what could happen over your side of the pond.

EDIT: And just airbnb things. Cheaper than hotels, less socially awkward than couch surfing.

iirc, Edinburgh Castle is also donation based.

Yeah, sorry, that's what I meant by "awkward", "anxious". It was 4am where I am in the world (nb: not the UK), so I was tired and made a mistake.

You can use technology to get around most unneccessary interactions. AirBnB, Uber, in London you might want to look at getting an Oyster card (and, if you have an iPhone, looking at setting it up with Apple Pay). Other things like ordering in a restaurant, I'm sure your friend can help with. Other than that, a single pint of beer should lubricate you up nicely!

What I meant is that... like, say when we go to Disney World, there's this show called Monster's Inc Laughing Floor. In this show, the camera will randomly pan to somebody in the audience and demand they tell a joke on the spot, or do a dance / silly face. It's all on a big screen, and they will sometimes make repeat visits to the same person, and linger for some time. You can tell the Brits in the room, because they are all sheepishly staring at the floor for the whole show, just praying. That shit would not go down in the UK.