Not a lawyer, but it is my understanding that anyone who wants to access your personal details needs your permission to do so. That should include soft credit searches. If you are concerned then I would contact a lawyer though, as they can give better advice.
~~https://money.stackexchange.com/a/35350~~
I would however think that you most likely consented to soft searches when you signed up for monzo
[Edited the above, as it’s incorrect, thanks Sufi:]
It appears that your consent is not needed to query your credit file:
Do companies need my consent to carry out a credit search
The [Data Protection Act] doesn’t actually require these companies to have gained your consent before they can carry out a search of your credit file as long as they have a legitimate reason for doing so and you have been told that this search is going to take place. If you have taken out a loan or credit card you will probably find this in the original terms and conditions that you signed.