Is it illegal in U.K. to drive with this windscreen damage?

It is illegal in the UK to drive a car that is not in roadworthy condition. If it would fail an MOT test, that makes it unroadworthy.

The windscreen part of the UK MOT test is defined in

• https://www.gov.uk/guidance/mot-inspection-manual-for-private-passenger-and-light-commercial-vehicles/3-visibility

This states

Condition of glass

damage in windscreen zone A more than 10mm in diameter
damage in the remainder of the windscreen’s swept area more than 40mm in diameter

Both the cracks pictured are longer than 40mm.

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The government website also says

Check your vehicle is safe to drive

You’re responsible for making sure your vehicle is always safe to drive (‘roadworthy’). It can be unsafe even if you have a current MOT certificate.

You can be fined up to £2,500, be banned from driving and get 3 penalty points for driving a vehicle in a dangerous condition.

When it comes to tyres, driving with an illegal tyre will most definitely be considered dangerous, and an offence.

"The crack does seem to be getting bigger rapidly."

A cracked windcreen might shatter and temporarily distract or blind the driver causing an accident. Whether a police officer will penalise you is not for me to say, but yes it is illegal to drive an unroadworthy vehicle (except by appointment after failing an MOT test).

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What can you do about it?

Your insurance policy may have a clause which allows you to claim for a windscreen repair without affecting the discount or claims history, with a nominal excess fee. Mine is £75.

The insurance company will probably want to approve the windscreen repair company. Some of them have mobile units that will replace the glass at your house. They can even do it in the rain, as they erect an awning.