Purchasing likely to sell out tickets for London Highlights

transport to London from Heathrow?

You don’t need to book ahead for this unless you want to save money on the overpriced HEX (which only goes to Paddington anyway)

For the Tube or Elizabeth Line just use your contactless cards or phone to swipe in and out.

I'd not book this early. What tour are you wanting to book thru Evan Evans?

I'd watch the official Buckingham Palace website to see how the ticket times are filling. Right now it looks like there is a lot of availability for September but you can easily keep an eye on the booking calendar.

https://www.rct.uk/visit/buckingham-palace

Where are you headed in London? Some hotel location might work best with the Heathrow Express. Some might work best with the National Express Coach and some might work best with one of the Tube/rail lines.

We are staying in the Borough Market area

Elizabeth Line to Bond Street, Jubilee Line to London Bridge. Short walk to Borough Market, use contactless to pay.

You don't need to worry about it now, I don't think, but you might take a look at the websites of the museums you're thinking of visiting to see whether they have special exhibitions planned that figure to be popular enough they might sell out ahead of time. For instance, the Victoria & Albert Museum has a Cartier jewelry exhibition scheduled to begin on April 12. All the tickets are sold out, as are all the tickets for weekend days in May except for one day. That's just an example. A lot of special exhibitions don't require advance planning, but things related to the Royals, famous musicions or impressionist art can be very popular.

Thank you all for the suggestions. Acraven I wouldn’t have signed up for what they add for exhibitions . I will

Having just looked at Evan Evan’s tours I can’t see one that I would recommend paying for. They are either ridiculously crammed when mostly you will just see the inside of a coach and a lot of motorway, or they can be done more easily and cheaply yourself.

Or if you take the Piccadilly line tube from Heathrow to Borough Market change onto the Jubilee Line at Green Park.

£13.90 on the Elizabeth Line/Jubilee line route, £5.80 on the tube

It is worth noting that no train in the UK can 'sell out'. It may be full and standing but tickets will still be sold.

For Evan Evans yes I think 6 months out is excessively early. You are just putting money into their bank for that time which may be hard to recover if your plans have to change for any reason. I've seen them boarding in London and they run duplicate vehicles as required.

For a market segment which is short on time and wants a guided tour they are doubtless good at what they do. Windsor, Stonehenge and Oxford you'd find it hard to do on your own in a day (by car or public transport). Of course each should be it's own day, but not everyone has that luxury of time (and the cost of the 3 days would equal or exceed the cost of the one day tour). It is maybe better to get a glimpse of all three, rather than not get there at all. They aren't my cup of tea at all, but nor would I dismiss them for those who need a quick introduction.
You can always go back to Windsor or Oxford on a later trip in future years, on your own account for a more immersive experience.