Welcome to the forums AHGPortsmouth ~
Let me try to clarify your question. You have relatives here in BC who have Canadian banking cards and you want to know if they can use their cards in England? Yes, they can.
There will be a few things to pay attention to. 1) they need to find a UK ATM that uses a logo that is the same as one on the back of their band debit card - either Plus or Cirrus. 2) they need to take out enough money to last them for awhile because each transaction will cost a transaction fee. A £100 withdrawal ONCE will cost less in the end than five £20 withdrawals.
I would also recommend that they contact their bank and CC company to double check what I have said above as well as to put a "travel watch" on their cards. There will be a foreign exchange charge (changes daily according to the bank rate) and a transaction fee per transaction. Each financial institution will have their own charges. The bank or the CC company may put a freeze on the cards if they think the charges made in the UK are not legitimate.
I wouldn't bother using travellers' cheques, they are really not the preferred way of using money out of the country. Some people say to use your CC for hotels, car rentals, larger restaurant meals and to use the debit card to take enough cash for daily purchases, entrance fees, coffee/snacks and tips. It might be a good idea to keep extra cash under their clothes in a money pouch on a neck strap.
I think it would be a good idea if you posted your question on the most appropriate forum in the UK (where are your friends heading to.) Those posters are the ones who will be the most up to date with the actual money details of their country.
Here is an article from the London UK forum that addresses these issues. Click on the blue print, then scroll down below the images of the currency notes for the information pertinent to your question.
tripadvisor.com/Travel-g186216-s601/United-K…