This site is helpful:
http://www.tyrereviews.co.uk/Tyres_For/Subaru/forester-2.5xt.htm
They are suggesting £75 or so per tyre - going to £300 for four.
It is a powerful car. Winter is coming. In my personal opinion, you are risking much more than a hundred pounds or so if you put super-cheap tires on this vehicle.
Others have suggested part-worn tires.
These are banned in many countries because of high risk of tires being dangerous. Here is what the National Tyre Dealers Association says about them
https://www.ntda.co.uk/useful-links/part-worn-tyres/
An alternative might be to rent tires
Asda will charge you a monthly fee:
https://www.asdatyres.co.uk/pay-monthly
A smaller operation called Gorilla does the same
https://gorillatyres.co.uk/pay-monthly/
Alternatively, BlackCircles offers a deal so that you don't need to pay for six months:
https://www.blackcircles.com/finance-options
In fact, if you search on "tyres pay per month UK" you get a number of options.
Your tyres are what keep you safe on the road. Good tyres will keep you safer than bad ones.
TyreReviews.co.uk will give you advice on which are good and bad,