The other solution is to wait until you get a comment or complaint from your neighbor..... which may never come.

You cannot pollard this palm - if you do, it will lose the growing point at the top and die. There's only two options - leave it in situ, they're not fast growers, or have it removed. Which seems a shame because it is attractive...

One person's "too high" may not be the same as another's... they can get a lot taller than that but nothing like a sycamore or ash...

But when and if it gets tall the leaves will be all at the top and the trunk will be bare so it may not take as much light as you fear.

It's an absolute beauty.

Thank you for all your considered responses. The information about its growing behaviour means I can now take a more informed decision, balancing out its impact on light with its aesthetic impact. It would be a great wrench to lose it, so I feel happier now about leaving it, and we'll see...
Thanks so much.
Diana