When are you allowed to wait on the yellow grid markings?

Explanation: The yellow grid markings are a box junction. They're used where stationary traffic is likely to block the junction.
You must not enter a box junction unless your exit road is clear. You can enter the box and wait when you want to turn right but oncoming traffic is preventing you turning. The right-turn exit must be clear.

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References: The Official DVSA Guide to Driving: Section 6, Highway Code: Rule 174, Know Your Traffic Signs: Page 67