What’s the reason for the yellow crisscross lines painted on the road here?

Explanation: Yellow 'box junctions' like this are often used where it's busy. Their purpose is to keep the junction clear for crossing traffic. Don't enter the painted area unless your exit is clear. The one exception is when you're turning right and are only prevented from doing so by oncoming traffic or by other vehicles waiting to turn right.

Category: Road and traffic signs

References: The Official DVSA Guide to Riding: Section 7, Highway Code: Rule 174, Highway Code: Page 116, Know Your Traffic Signs: Page 67