Yes. The UK is a member of the Universal Postal Union and is obliged to accept them (although not obliged to sell them).

According to their website the Royal Mail stopped selling them from December 2011, don't think they can be used anymore in the UK..

I am not certain, but I think IRC's no longer exist. You can go on the Royal Mail's website and buy postage, but whether they accept foreign currency or not, I do not know. One way would be to send the recipient a payment via 'PayPal'. All you need is their email address and you can send them the money to buy the required postage.

Yes. As Jo W has already pointed out, the UK is a UPU member so has to accept them.

I heard that that Royal mail doesn't sell international reply coupons anymore. I am sending a letter to someone in the uk and I have to pay for the return postage. I cannot acquire any uk stamps (i live in canada) so I REALLY need to know if the uk still accepts them (even though they aren't sold anymore)