Hi Halfpint and welcome to the forum.
First I am US based so have only passing knowledge of the UK training and certifications. However I have chartered in the Bahamas
a couple of times when I was between boats and can offer some advise there.
If you have little or no experience on boats then a good instructor and training program is a great way to get started. However my experience with charters is that they care more about your abilities than any training, certifications or licenses you may have. I am mostly self taught as a sailor and never took a formal class but have not had a problem at all the couple of times I chartered. The first time I sat through a briefing with the charter operator and based on my responses and questions he was completely satisfied and let me go with the boat. The second time, many years later and with a bigger company, their standard policy was to require any new customers to take a written test and mail it in before the charter was approved. Same results, the answers on the test and a meeting with the operator before departure (that insured I was the person that wrote the answers) was sufficient.
In some cases if there is any question the charter company may require you to take them out for a test drive so they can observe in person. Any doubts and they can require you to pay for a professional captain
to go along.
So, don't get me wrong, the classes
and certifications will be good and will help but at the end of the day most operations will want to see what you can really do.