Flying to Geneva- going to Val Thorens

Is there border checks and are you allowed, as a UK citizen, to fly to Geneva and then go to french alps?

No you are not currently allowed to cross into France and ski in the French Alps.

No, France is still effectively closed to UK holidaymakers. If you already have a flight into GVA then the easiest alternative is to ski in Switzerland. The obvious choice is Verbier or if accommodation is there is outside your budget, then a linked satellite like La Tzoumaz or Nendaz would be cheaper. If you self-cater then the accommodation and grocery bill should be comparable to any other large Alpine resort. For the lift pass, last time I looked, the Verbier day price was £65/day for an all-areas adult day pass, around £55/day for a period pass for the linked 400Kms of pistes in the 4 Vallées.

I wouldn't hire a car. Prices at GVA have become prohibitively expensive. You can get from inside the GVA terminal by train/télécabine to the centre of Verbier in around 2½ hours - the timetable is on the SBB site. A transfer by public transport is arguably easier and less hassle anyway.

The only caveat is that Switzerland has already imposed and then rescinded mandatory quarantine on UK arrivals and there's no guarantee this won't happen again - but that's going to be a risk wherever you go. Currently, they at least offer free lateral flow testing and certificate at many pharmacies, so your test to return won't cost you. Although again, this is a reversal of an earlier charged test.

Are you sure that the tests are free to holidaymakers? I have a feeling it's only for Swiss nationals with symptoms?

Lateral flow / antigen tests are currently free in Switzerland
https://www.bag.admin.ch/bag/en/home/krankheiten/ausbrueche-epidemien-pandemien/aktuelle-ausbrueche-epidemien/novel-cov/testen.html

Might be different in practice, but Switzerland Tourism confirmed to me:

Rapid antigen tests are free of charge in Switzerland for the purposes of obtaining the Swiss Covid Pass (not for return travel)

I haven't found an official source that supports that statement. Covid testing and the issuing of Covid certificates is the responsibility of the Swiss Federal Office of Public Health (FOPH). Switzerland Tourism is a publicly owned corporation that is responsible for promoting Switzerland as a holiday destination. It is not an official source for information on the validity of Swiss Covid certificates.

If you click on the link to the FOPH website that I posted above you will see that it says "The federal government covers the costs of nasopharyngeal rapid antigen tests for everyone". If you look at Switzerland Tourism's Covid-19 travel conditions webpage https://www.myswitzerland.com/en-ch/planning/transport-accommodation/travel-to-switzerland/where-and-when-is-what-open/ it also states that that rapid antigen tests are currently free of charge. There is no mention of antigen test certificates issued after negative free of charge antigen tests not being valid for "return travel" on either of those websites or anywhere else as far as I can see.

Can anyone please tell me if I am flying from Ireland to Geneva but am transiting straight to Val Thorens (I have proof of transit and accommodation), do I need a pre departure test? It is a requirement of Swiss but not France..

I had Covid 2 weeks ago but my recovery cert won't be here on time. I am out of isolation but still possible that I will test positive.