Can I buy Irish stamps anywhere in London - need to enclose a SAE in letter from UK to Ireland?

[AbsentmindedWoman]

I'm so dim it has only just occurred to me that I can't blithely enclose a stamped addressed envelope from London, because the stamps won't be Irish and the SAE needs to be mailed from Ireland.

Is there somewhere in London that sells Irish stamps? I'm not sure what to do - it's for an Irish police certificate.

There used to be a thing called an International Reply Coupon, but that's been discontinued by Royal Mail.

Best alternative is here:

www.anpost.ie/AnPost/IrishStamps/Shop/