London, Scotland and Ireland in 5 days?

Yes, choose one. I would choose London.

I would just do London since you have never been there. London is a wonderful city that is easy to navigate with their great transportation system. The city is vibrant with many nearby sites for possible daytrips too. Most London museums are free. Also, getting to and from hotels to train stations or airports take up a lot of valuable sightseeing time.
I have been to London, plus other places in Europe, yearly for the past 7 years and I never get tired of spending more time in London!
I have been to Wales and Scotland but not Ireland. Would you be renting a car for Wales and Scotland? Or take tours? I still say London first then return with more time to see other parts of Great Britain.

London. Tons to see and do. Something for every taste. No time wasted on hotel changes. Many day trip possibilities.

In 5 days you need to pick one country. You can't possibly see Ireland, Scotland, and England in 5 days. You would do nothing but be spending time getting from one place to the other. If this is your first trip, I would tell you to go to England. With only 5 days you could spend all that time just in London, but I would suggest you go to London then choose one other destination in England, such as York (north) or the Cotswolds. (west). plan on going back to Europe but next time give yourself more time on the ground.

Go to London. As stated, lots to do.

If you wanted to, you could take a day trip by train to places such as Oxford, Cambridge or Bath to see at least a little of England outside of London.

You can do these on your own or take a guided tour. London Walks is a company I can recommend for these trips as well as their London walking tours. Tours are given on different days so see if there are any that match your schedule.

Ditto - just London - so much there to fill 5 days easily.

Check the National Trust UK online site for the Greater London area. You will see plenty of places to visit in London and the environs. One of my favorite places to visit is Fenton House in Hampstead. It is a Georgian House with a walled garden and a lovely exhibit of antique musical instruments. The street markets are in many parts of town on different days. You can go treasure hunting or just be part of the fun. There are lots of theatres with some discount tickets available. Take a ride on the Thames. Go to Greenwich and stand on both sides of the World at the Observatory. You will wish you had more than 5 days.

London, London, and did I mention London.

5 days there will barely scratch the surface but u can make a dent.

GREAT city. Enjoy it!

London, without a doubt. There's enough to do there to keep you busy for a month!

You may want to buy the Rick Steves London guidebook, and start reading to see what interests you.

Likewise, we can make many recommendations here.
I recommend all the museums, many of them free.
The British Museum
The Victoria and Albert Museum (V&A)
The Museum of London
The National Gallery
The National Portrait Gallery
Churchill War Rooms
The Imperial War Museum

Pick one destination and then consider a day trip on day 3 or 4. Personally, I would pick London but it's your trip and Scotland or Ireland would be fun!