The main medications currently are semaglutide and tirzepatide but they then have branded names as some countries (possibly just America but I'm not sure) had to have different names for insurance purposes. An insurer might allow an American patient one medication to treat their diabetes but not the same named medication for another patient without diabetes but who wanted it purely for weightloss.
Semaglutide is branded Wegovy or Ozempic.
Tirzepatide is branded Zepbound or Mounjaro.
In the UK we have Wegovy and Mounjaro. Both are GLP1s but Mounjaro also has a GIP so two ingredients that mimic natural hormones in our bodies to help us feel sated after eating and fuller for longer.
There are other drugs on the way -
Retatrutide has three ingredients but it's not legally available anywhere yet. Lots of people currently taking it are obtaining it on something called the grey market, essentially not legal.
Foundayo or orforglipron is a pill which is available in America but not yet approved in the UK. It's very similar to the Wegovy pill but somewhat easier to take as you don't need to take it on an empty stomach or abstain from eating or drinking for 30 minutes.
Mounjaro is only available as injections.
Wegovy is now available as injections or pills.
I don't think many people are able to source weightloss medication via their GP. The people who decide on where money is spent have set criteria to even allow a GP to prescribe and it seems to differ by where you live - the postcode lottery.
All UK online pharmacies have the same procedures and guidelines to follow as the ones on the High Street. They are regularly inspected and if they fail the inspection can be closed down.
If I wanted to start on the medication I'd choose the Wegovy pill because it's a little cheaper and easier than injections. I'd use the comparison site mentioned above. It does usually work but you do have to follow the links. The people who run the site are quick to correct any broken links and update the prices daily.
Hope I've covered everything.