The fuel tank has space built in for thermal expansion.
On older cars (like the mk3 Golf) there was a nipple in the fuel filler neck. If you pushed it you could fill that expansion area too (up to 10 litres after auto shut off), if you did that in the morning with very cold fuel and then left it in the sun at 40 degrees then maybe you'd get an issue.
Just filling it until the pistol shuts off and then putting another litre or two in you'll be absolutely fine.
Also don't forget; a UK heatwave barely makes spring in the rest of the world