Seamaster replacement strap

Hello,
My son has recently inherited a gold Seamaster watch with a gold strap/bracelet. It is much too small for him. His wrist measures 21cm!
We cannot begin to afford a gold strap. We would like a sort of tan brown leather with lighter stitching and a gold coloured buckle. I am tasked with finding the strap. The bar bit where the strap attached is 18mm wide but when I look at straps they talk about all sorts of fittings and I am so confused.
Can anyone advise me please?
In an ideal world a strap for £50-70 is our limit, and if we could get our hands onmone in the UK before a wedding in a week even better!
Thank you very much for any help!

Check out Watch Gecko and Watch Obsession. Lots of nice straps from both and they are in the UK.

You want a strap with a width of 18mm. Straps are straight across at the end that connects to watch so you don't have to worry about whether it fits. An 18mm width is pretty common so you should have lots of choices. I also recommend getting a strap that does not have padding between the two layers of leather. A padded strap can work for a larger watch like a diver, but do not go as well with a dress watch like the one you have.

I would also add a springbar tool to the order so you can remove the bracelet that's on the watch now and put on the new strap.

There are lots of tutorials on YouTube on how to change a strap if you've never done it before.

Thank you very much for your helpful advice

Any watchmaker have straps in stock and will change it for you. I think a bit of red goes well with a gold watch.