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1) Yes - You have 96 hours to take the test before depature time, see https://www.emirates.com/uk/english/...ourists/#82485 you need to have a test to depart India for Duabi even if your only passing through. You should use a lab EK certifies. The same rules apply for EK and EY.

2) I would assume no, the UK does not require tests at present to enter the UK. If you enter Dubai, this is where it changes, EK does not require a test if your destination does not, so the UK does not require, EY seems to require a test to depart regardless. So your better on EK if your entering DXB otherwise go with whatever suits your timings and budget