I don’t think don’t need to have everything figured out or give out all the specifics, but you should give out some specifics to show you have a plan, not just a “wish” if that makes sense. If you already started a startup and plan to start another one in the fintech space, you could talk about how you will apply the lessons you learned the first time around on your new startup in the UK. You can talk about why the UK is the best fit for your idea, or how your startup will fit into some specific aspect of the UK Fintech industry.
My advice is to be specific about your goals and show that you have a vision and that you know the field.