I’ve been behind on paperwork for my UK limited company and haven’t submitted the annual filing or tax return yet. I’m getting worried—what actually happens if I don’t submit them at all? Are there fines or serious consequences I should be prepared for?

If you don’t submit your UK limited company’s Annual filing and Tax return, your director and company profiles will be flagged. And because your director and company profiles are flagged, you could get in trouble with the European Union. The troubles you could get into are-
All accounts and payment gateways (PayPal, Wise, Stripe) in the company’s name will be closed.
The UK Tax Department will try to deduct the penalty money from the stock balance in the company’s bank account and payment gateways.
In the future, any personal EURO account could be closed, or the account reopening applications could get rejected.
Visa processing in any country in the European Union will become complicated.
To form you UK company seamlessly: https://businessglobalizer.com/services/uk-company-formation/