You could wire this together yourself. That was never the question.
You're capable enough to bolt a ticketing tool to a CFP tool to a spreadsheet, and
plenty of organizers do exactly that. The real question is whether being the maintenance
crew for that stack is how you want to spend the event. A chef who builds a cooking
robot has stopped cooking — your craft is the conference, not the glue between the tools
that run it.
Fomomento carries the plumbing: UK and EU VAT kept correct as the rules change, payouts
reconciled through your own Stripe account, and gap-free invoices your accountant and
tax office will accept. The control stays yours; the maintenance doesn't.