Tyre pressure light is on but tyres are fine?

So, my Ford Fiesta ST-Line 2016 has its tyre pressure light on and comes up with "Check Tyre Pressure" on the screen. I have checked all 4 tyres and they are fine. The car has been rarely used within the last 4 months due to injury but I'm concerned that there's something wrong.

Can anyone help?

Where Can I Buy Alloy Wheels Not Online?

I need new alloy wheels as my 2009 Ford Fiesta (16inch 4 studded) ones are getting old and are a bit banged up. I don't trust these online websites and actually want to see them before buying them to make sure they fit.
Also it would be handy to buy them and have them fitted in one place.

Where Can I find A Shop In Wiltshire (England) That Would Sell Them And Have Them Fitted?

What Alloy Size Does My Car Need?

I know nothing about cars, but need new alloy wheels as they are a little banged up, but have no idea what size I need.

I drive a 2009 Ford Fiesta 1.6l Ti Diesel.