Hi @DianaB! I am happy to help.
Your question makes sense. If you never had orgeat, it is a good idea to try a commercial product first so you have some kind of benchmark to compare against. My first bottle of orgeat was Torani (which isn't horrible but I wouldn't recommend). The first time I made my own I knew I could never use the commercial stuff again! But nowadays there are a lot of brands that are made on a small scale and are actually quite good.
I would vote against Monin. It is rather indistinct and won't give you a true idea of what orgeat can be like. I think Giffard is a better bet if you don't want to go through the trouble of making your own. We have access to several small artisanal brands in the US, but I am not sure if there is an equivalent in the UK. I browsed on amazon UK for a bit and didn't come across any.
Difford's, a respected cocktail website which is based in the UK, has a bunch of orgeat reviews. They also prefer Giffard to Monin.
Also, I am not sure about shipping costs, but a place like The Whisky Exchange is a much better source (selection and price-wise) than amazon for cocktail-related purchases. You might want to check them out! Master of Malt is another good one, but they only seem to have Monin.
Good luck!
Out of curiosity, do you have a specific use in mind for the orgeat? A Mai Tai maybe?