Disclaimer: yours won't be fiscal advice. People involved will consult a professional before doing anything. This is just to gather some preliminary information.
- you want to buy a house or a flat, in UK
- you will get a mortgage in GBP, but you'd pay the down payment in EUR
- you think that there are fairly high chances that the value of houses in UK and GBP will drop
- the whole point of this question is actually buying the house/flat, renting is not an option by definition
What can you do to mitigate that risk?