Power of attorney - UK or Belgian?

Go and see your local notaire.

Your will is in UK implying you are in UK. Therefore the power of attorney is in the UK as well, regardless of where your niece lives.

You should definitely go and talk to a notaris.
A UK will will not automatically apply to your property in Belgium. The advice which I was given is that I should have a will in both countries - or be intestate if that is my choice - to be sure that what I want to happen is what actually happens.

As "J" notes above, and I missed in my first answer, is the fact that bobk100 hasn't specified where they live (and perhaps more importantly, where any assets are based).

If bobk100 is based in the U.K. they should consult a solicitor in the U.K.

If bobk100 is based in Belgium they should probably consult a notaire in Belgium (notwithstanding the existence of a UK will).

In any case, it's also not clear what bobk100 wishes this power of attorney for, if it's for financial matters, or health matters for example.

And finally, it's important to note that a Power of Attorney is completely different and separate from any will. Executing a will is the job of the executor(s) of the will.

Even if the PoA and the executor is the same person, they will have different responsibilities and liabilities depending on whether they are acting as the executor (because bobk100 is now dead) or as someone with a Power of Attorney (because bobk100 is alive but requires someone else to act for them).

OK that simplifies things considerably.

1) You are in Belgium
2) The person you wish to appoint as PoA (irrespective of their nationality) is in Belgium.
3) Your financial assets are in Belgium
4) Your healthcare is presumably here in Belgium.

Speak to a notaire.

Finally - you're getting two issues mixed up and confused here, your will (which deals with you and your estate once you are dead) is completely separate and distinct from who you may or may not appoint as a PoA (which is for acting for you while you are alive).

I can only repeat that you must see a notaris. And I speak from careful experience of dealing with my late wife's property and preparing for my own future.
The internet - this site or otherwise - is not the place to seek advice about something so important.
The rules regarding what you may leave in a will and to whom you may leave it are totally different between UK and Belgium and, when the person in the UK who has been promised that they will inherit from you, discovers that Belgian rules don't permit it, it's too late.

http://www.berquinnotarissen.be/nl/
This notary based in Brussels is expert at inheritance law, especially for UK/Belgium questions.
There is no charge for discussion with them, nor for advice. They charge only when they actually draw up papers. Prices for PoA and wills are very reasonable..

You need the PoA set up under Belgian law.
Get advice on a Belgian will as well - inheritance tax is going to need careful planning.

As you don't have property here, life is much easier. A house has to be left to your children (if you have any) and a wife has the right to continue living in it after you die, until she dies.
For your cash, if you die tomorrow, the estate pays maximum tax. If you give away some in advance of death, you can pay much less depending on how long you survive. Sliding scale.
The notary I advised above will explain all the options at no cost. Any notary will do the same. The advantage of the one I have mentioned is that they have specialists in international advice.