I'm an Indian citizen living in India and fully vaccinated with WHO-approved vaccine.
India has air transport bubble for international flights. I'm planning to travel from India to Barbados for a week which is outside this bubble. The flight would most probably be from the UK, which is inside the bubble.
Is it allowed to make this journey as I'll be leaving and entering India from a country within a bubble?
Per the guidelines you yourself link to, bubble flights to the UK are supposed to be for:
Any Indian national holding any type of valid UK visa and destined for UK only.
If you do not hold a visa for the UK, you will thus not be allowed to board. Even if you do hold a UK visa, you may experience problems if your itinerary indicates you're only transiting instead of being destined to "UK only".
However, if you do have a UK visa that allows multiple entries, I don't see anything that would stop you from booking one India-UK "bubble" roundtrip and nesting a UK-Barbados roundtrip within that. It's not clear to me if you're supposed/"allowed" to do this, but that's a separate question.
All that said, given how difficult travel continues to be these days, I suspect you would find it much easier and cheaper to travel to a tropical bubble destination like the Maldives or the Seychelles instead, since both can be reached on direct flights and travel to both is 100% permitted.
