Upvote:3
The relevant web page (on gov.uk website) does not say anything about where you need to apply from: link 1, link 2.
That said, it's important to think about the processing times and how long you are planning to stay in Geneva. UK standard visitor visa can take several weeks to process; if you apply from Switzerland, you'll be expected to attend the appointment in Switzerland and pick up your passport with the visa in Switzerland. Moreover, you'll have to submit your passport as part of the application - and may not get it back before you have to fly back home.
So, while technically there is nothing mentioned on website about having to apply from your home country, you should consider the implications of such decision.
In either case, you can always contact UKVI and ask for clarifications - they are usually helpful, but it's not free - contact information in English for those outside UK.