You will see prices for Mounjaro on monj.co.uk and Wegovy on sister site igovy.co.uk. Both only show GPhC pharmacies. There is also an option looking at smaller local pharmacies.
As you have already read, in trials, Mounjaro was the better medication. If price is not a barrier I would go with Mounjaro.
If you are using an online pharmacy, once you place an order they will do a consultation with you. Some are more thorough than others, but if you have any medical conditions or are not eligible for the medication it should get picked up at this point. You will also need to send in photos, perhaps do a quick video consultation to get your height, weight to confirm BMI, show photo ID. You are unlikely to talk to them unless you have an issue.
The pen is easy enough to use if you follow the instructions, watching a pharmacy produced video is also useful to see how to use it before your first time. Stick to the instructions until you understand what you are doing.
If you get unpleasant side effects (nausea, constipation, diarrhea, acid reflux etc) you should be able to speak to your provider by email or phone, but responses can sometimes be a day or two, or you can speak to your local pharmacy for support on how to manage them. Online pharmacies who specialise in these medications are likely to have more knowledge of them.
If you have a concerning, or urgent medical issue (pain etc which is rare, all medications have uncommon side effects) your provider who whether local or online only works office/pharmacy hours most of the time would not normally be your first point of contact, it would be the usual NHS services.
Other things you need to know? Most you will know from previous diets that you couldn't stick to! Mounjaro does not directly cause weight loss your calorie deficit will (work out your TDEE). To work with Mounjaro you will need to change how you eat - most eat protein focused meals (to preserve muscle), lots of veg/salad, as few UPFs as possible, reduced carbs, minimal sugar/sweeteners. Mounjaro makes it easier to stick to this long term (I am 16 months in and still don't miss or think about snacking etc). In the beginning it may give you appetite suppression, and you could find it difficult to eat, but that tends to wear off and you are left with reduced food cravings and portion control (if you continue to work with it). Losing weight fast is not healthy. ChatGPT is good for help with menus - use an app for the first few weeks to check your calories and macros.
Most of the (non medical) things you might need to talk to someone about, you can get more experiences from forums like this or if your provider has one their own chat forum, or perhaps reddit than from the pharmacies themselves. There are not many facebook groups anymore as facebook kept shutting them down.