safety of London tube very early in the morning and train to Edinburgh

It will be very safe. However, you will need to make sure the tube will get you to the station in time. With the exception of the ”night tube” on Friday and Saturday nights, the tube does not start running until after 5am. Depending on where you are staying, you might not be able to get there in time.

Check the journey planner on tfl.gov.uk for the day of the week you plan to travel to get an idea if it is possible.

Rooms for 3 adults are challenging to find. I’d try checking booking.com or search for a double and a single.

Black cabs are relatively cheap and at that time of morning it would be quick. I would take a cab just for convenience.

Is booking.com a better option than airbnb? I did not know if there are problems with flats on booking.com?

I’m confused.

You say you are staying with friends in London for 5 days. Then you say you haven’t booked accommodation and are open to suggestions for 3.

Why is the Kensington neighborhood important?

Lastly, your concern for safety. I’m decades older and 2 years ago walked at nearly midnight from the Richmond Quadrant to the Premiere Inn on Lower Mortlake Road.

There are Premier Inns nearby; Premier Inn London Kings Cross OR Premier Inn London St Pancras Hotel.

I stayed at Hotel 63 on Bayswater Road across from Kensington Gardens and loved it.

It was 1/2 block away from the Lancaster Gate tube station as well as many buses.

https://hotel63.co.uk

Very unique bed and breakfast.

I am 78 and travel solo, Felt very safe in London but if it is in your budget, you might want to take. a taxi to get to King's Cross.

My suggestion for your trip to Kings Cross....take either a taxi, Uber or pre-arranged car service. (Your hotel will probably be able to arrange a taxi or car service.)

If you can manage with two beds--a double and a single--then look at Premier Inns. If you need three beds, then look at Premier Inns with family rooms.

There's a very simple way to cut down on your concern. Forget Kensington, which is miles away from King's Cross, and find a hotel by the station. There are three Premier Inns within easy walking distance of King's Cross, all of which will provide a perfectly comfortable room for the night at a sensible price. There are also many more options at all price points.

If you are flying in to LHR you can take a direct Piccadilly line train to King's Cross in about an hour or from LGW a Thameslink train will take you straight to St Pancras, which is across the road from King's Cross.

I would also say stay within walking distance of King’s Cross Station.
It’s a safe area.
That’s where I prefer to stay, as I also get the train to Edinburgh after my few days in London each trip.
I was just there in December and was thrilled to find out and use a number of different bus routes from there to easily get to different parts of London.
Of course you can use the Tube too.
The Citymapper app works brilliantly for bus and Tube routes.

I won’t tell you where I stay, as I had a mouse in my room one night there in December!
I will still go back there next time though, as it’s super clean and friendly otherwise.
Mice are rife in big cities , they are there even if you can’t see them.
It was just my bad luck he came out of hiding!
Just the hazards of big cities and very old buildings.

Premier Inns are just along the street from the station.

If you need three beds, then look at Premier Inns with family rooms.

This won’t work for three adults. Premier Inn only allows 2 adults in family rooms. You would need 2 rooms at a Premier Inn.

There are a number of Accor Hotels (Ibis, Novotel and Mercure) in central London which do Triple Rooms. Look at their website for your dates, but around that time I can see them for around £200 a night per room which isn't bad.