COULD I MAKE A LAYOVER WITHOUT VISA?

Question

Dear mr / ms,

I am from Ukraine, but I live in Spain.

I would like to take the flight from Santander to London, make a layover and then get the flight from London to Lviv.

The question is that my nationality is ucrainian. Is it possible to do the layover in London without visa?

If the answer is yes, could I do the same on my way back? Could I go, from Lviv to London, make te layover and later take te London - Santander flight?

Thanks in advance