This is the monarch butterfly caterpillar. It feeds only on the milkweed plant for the plant contains poisons that the caterpillar ingests and accumulates. These toxins are transferred to the butterfly and it is a form of aversion defense for it. The monarch butterfly has a very interesting life history which you may like to familiarize yourself with.
Now, either your daughter lives in North America or she has taken a photo of one of the monarchs that have been transported by ship and have invaded the UK. The monarch migrates to a particular area in Mexico for the winter and part of the migratory fly route is along the east coast of the USA so they might land on a ship close to the coast for a rest and then find themselves in port in the UK later. I assume that milkweed grows in the UK also though I am unsure whether it is a natural grows or intentionally planted in gardens to attract this butterfly.

Is Milk Thistle Milk Weed ? Monarch Butterflies fly across
the Atlantic Ocean and have been found in Cornwall.

You're correct of course Loosestrife about my daughter's hometown. She lives in San Diego. And thanks for the info. But it's a helluva long way to fly across here.

They can fly and glide along on their fat reserves for about 1,000 hours. The ones that are the most successful in getting across the pond are the ones that bummed a ride on a freighter no doubt.

If we see them here, its usually the adults, although Kew Gardens found a couple of larvae on milkweed there, and removed them to their butterfly house, where they matured. They are rare visitors to the UK, but they do appear sometimes. Very striking larvae...

Diane, its unrelated to the milk thistle and is not native to the UK.
The caterpillars are very dramatic!