If they can declare independence, why can’t Taiwan?
Because Britain won’t bomb Barbados?
Barbados has been an independent nation since 1966. 15 former British colonies* recognise the British monarch as their non-executive Head of State. That doesn’t in any way affect their independence.
*Antigua and Barbuda, Australia, [Barbados], The Bahamas, Belize, Grenada, Canada, Jamaica, New Zealand, Papua New Guinea, Saint Lucia, Solomon Islands, St Kitts and Nevis, and St Vincent and the Grenadines.