Something like this
https://amzn.to/2EZeUVp is about £7 for 2. They are mounted from the top as to me they will be less prone to leak as the hole is higher in the tank.
If you need a 12v relay
https://amzn.to/3h3Au8N at under £7 would work. You could use this to switch a 240v or 24v pump if necessary. Only 'play' with 240v if you know what you are doing as it can 'bite' in a big way!!!
For a solynoid valve
https://amzn.to/3bm7x6u is about £9 works on 12v takes 100psi and has 1/2inch barbs.
I am assuming you have 12v to power the solynoid? It's a normally closed which means without power it shuts off the flow.
The float switch can be setup to break the circuit when the level rises which would work to break the 12v to the solynoid valve.
Hope that above gives you an idea. There will be lots of other types but Amazon is easy and has lower free p&p limits than some others.