As an aside, apparently, UK airports have sniffer dogs that detect cash and if you are found with more than £1000 on you, you can be asked to prove where it's from. Only heard about that a few years ago.
That's not true.
There are no restrictions if arriving from or travelling to another EU Member State and if arriving directly from or travelling to anywhere else, you only need to declare if carrying more than EUR 10,000.- or equivalent.

I'm not talking about restrictions entering, I'm talking about additional security checks you may be subjected to. Check out the "Proceeds of Crime Act 2002" which concerns people exiting the country with over £1000 cash.

What is being referred to is called FINTRAC. Virtually every country in the world is part of that system. I work for an international financial services company.
In Cuba, a house cannot be purchased in cash. The money had to exist in a bank account. There are specific notaries that only do house transactions. You have to go through them. You don't have a choice. They take care of the paperwork, including the transfer of the payment which is in the forum of a cheque issued by a Cuban bank. They also collect the tax on the transaction.
The notaries are quite busy these days. Getting an appointment could take up to 2 months in places like Havana, for example.
How you get the money into Cuba is a separate issue but it does have to, eventually, end up in a Cuban bank account.
Though surely that is the official payment and any unofficial part would have to be cash ?

Of course.... Everybody hates to pay taxes, no one more than Cubans...lol
Yes, you can sell a house for $50k... The seller and buyer agree to, officially, do the transaction at $30k, the difference being transferred privately. Everyone pays less taxes. Still need the $30k in a bank account.
The government, though, has not been idle on these tricks. Houses now get assessed for value, much like here for purposes of annual property taxes. I have seen some evaluation rolls. I do admit that the government hasn't gone heavy handed with the assessments but it still puts a 'bottom' to what you will pay in transfer taxes, regardless of the stated selling price.

Hey.....I have entered Cuba several times with between $7000 and $10,000 canadian on several occasions and no
body ever asked me how much I had. Cash hand to hand is always the cheapest as far as taxes and commissions go.
Another way is get yourself a CIBC Visa debit card, front load it with whatever you want, then withdraw it from any ATM or inside the .bank at the teller as you need it.......that way you don't have to carry the cash with you while travelling.

The only thing to consider is, if they ever check you and find that amount of cash, undeclared, are you prepared to lose it ?

Don't Canadian banks flag you guys if you start taking out large sums of money from the bank?
So what? If it's your money your entitled to take it out. If they want to check whether it's due to illegal activities that's fine too. At least provided it is meant for legal transactions. Buying a house seems pretty legal too me,though paying in cash sounds dodgy. I don't know Cuban law, so don't know about foreign ownership or loopholes to get around restrictions.
As far as I know loads of such flags are raised without any issue's, it's just a formality unless of course something dodgy turns up.