Hi @w.fly or anyone else who is interested,
I’ve finally worked out how to apply tax rates to Stripe subscriptions through the Bubble API Connector 
1/ If you don’t have an API for Stripe setup in Bubble then thats the first step you need to take, using the API Connector. I already had it setup to perform other tasks like deleting cards or subscriptions.
2/ Once you have the Stripe API setup in Bubble you have to create a new API call ‘Add another call’ with these parameters. Remember to use real values so that you can ‘initialise call’ and check that everything works.
3/ Create your tax rates in Stripe. When you create new tax rates you can also decide if they are inclusive or exclusive,
eg.
Exclusive
1€ +0,25€ (25% tax) = 1,25€
Inclusive
0,75€ + 0,25€ (25% tax) = 1€
4/ Create an option set for the different countries and add the tax ids from Stripe.
You can copy the option sets from this page and then apply the tax id’s from Stripe,
4/ Create a new action in the new subscription workflow so that you can apply tax. If you are only adding one tax rate then you can literally add it as a static id, if you need to apply tax rate depending on country of customer just add the dropdown where user selects the specific country from the country option set. Remember to apply the field ‘Stripe-tax-id’ and not Display.
Thats pretty much it. When the workflow creates a new subscription for a customer it will apply the tax rate according to the country the user selects in the dropdown.