So, your BMI will need to be 30 or higher. Or, 27 or higher if you have a weight related health issue like high blood pressure. Or, 27 or higher if you are one of a list of ethnicities where obese is defined as 27 plus.
In order to get the weight loss injections online you will need to know your weight and height, as they will all ask for that information in order to work out your BMI. They will also want some combination of dated photos that show your figure, a dated photo of you stood on scales, or a short video consult to check those things.
The two injections available are Wegovy or Mounjaro. Wegovy is the UK branding of the same drug that is in Ozempic, for weight loss specifically. Mounjaro is a similar but different medication that is newer, it has two actions that help with weight loss and has been shown to produce more weight loss over time than Wegovy. You should read up about both, before deciding which might be suitable for you. You can find information about both on the websites of pharmacies that offer them eg Asda, Boots, Superdrug, Morrisons, LLoyds, Ashcroft, etc etc.
Prices vary a lot, so check out a price comparison site like Monj.co.uk to see what the range is.