NCLT - non-circulating legal tender
Here's what the RCM has to say..
Can I redeem a collector coin at a bank or use it as currency to purchase goods or services?
All coins manufactured by the Mint are legal tender. However, unlike Canadian circulation coins, collector coins are non-circulating legal tender (NCLT). As such, these coins are not intended for daily commercial transactions and accepting them as payment or for redemption is at the discretion of businesses and financial institutions.
The Mint has a process in place to reimburse financial institutions the face value of redeemed NCLT coins, once they have accepted them from a customer and returned them to the Mint. In the event a bank branch is unaware of this procedure, customers are advised to contact the Mint with the coordinates of the bank branch, which will take steps to inform the branch of the redemption process.
As collector coins can only be redeemed at face value by businesses and financial institutions willing to accept them, it is recommended that individuals wishing to sell a collector coin first consult with a coin dealer, who is more likely to offer a price above face value.
Slightly different, but here in the US, a business can refuse to accept a $100 bill should they choose (due to counterfeit risk). Is a $100 bill not legal tender??
How about handing over 10,000 US pennies for a $100 purchase?

Is a penny not legal tender??
For a bank, they have the knowledge (hopefully) & experience of handling real $100 bills.
For 10K pennies? They'll accept at the bank, but only after telling you to go get them rolled first. Basically they don't want to deal with the hassle of all those loose pennies.
And the same goes for those UK banks, methinks: They just don't want to deal with the *hassle* (i.e., time/expense) of exchanging those commemorate legal tender coins for "real" money.
Yeah, those 20-for-20 or 100-for-100 or whatever is just another way for a mint to charge some people a much higher premium than they could have otherwise gotten..
I steer clear of them.