Can I buy a Steam Card in the UK and use it on my account?
Yes. See the incorrect answer above/below this answer. Otherwise, this backup answer by me should work as well.
When you use your Steam Wallet Card, it'll automatically convert its currency into which the store you're currently using's own currency.
Since you're in the Netherlands' Store, your Pounds (from the card) will convert into Euros (store currency).
Can I buy [Insert Game Name] at a cheaper price; more specifically, the UK's prices since I'm there?
Yes, only if you switch your store location. To do this, simply go to your accounts page and select "Update your store country".
Only do this while you're in the UK.
This unfortunately means that any remaining wallet credit will be converted from euros to pounds, and some of its actual value may be lost due to conversion rates*. But on the plus side, the incoming money from your UK card will be 1:1 because it's the same currency.
*For example, if the rate is GBP:USD=X was 1.5, the effective rate that Steam will change Pounds into US Dollars will have a tax, so instead of getting $1.50 USD per pound, you'll only get $1.45 USD per pound. This value differs per currency and from minute-to-minute^.
^I seriously doubt that Steam will change its rates from minute-to-minute. AFAIK, it's more like daily.
Is this legal, and will I risk getting my account blocked?
Yes, it is legal. However, to stick with the Steam Subscriber Agreement, I'd recommend switching back to your home country's store after returning home from the UK, and to not use your Netherlands payment method (ie. credit cards) as it is against the SSA to purchase from a store that you're not in. (ie. US store from Australia, despite both using the same currency, and oddly enough - In Australia, we can choose to switch between US and AU stores)