As stated by several peple, speed rating determins how a tyre behaves with regards to heat disipation and other factors. I had a conversation with Continental about fitting V rated tyres instead of W because of better availability. They said don’t because when testing was carried out the engineers may have determined that the W was benificial over V.
Winter tyres will also have been tested and approved, sometimes with a higher load (and inflation pressure) to acomodate the differences in speed rating. I found that a set of 91H winter tyres ‘squirmed’ more that 91V.
Many insurance companies are not bothered as long as you fit tyres that can cope with the max speed of your vehicle or because the max speed in the UK is 70mph.
Go back and get them changed unless the fitter/ manager was on the team that approved tyre combinations when your car model was designed and tested then they cannot know or say it is safe.
edit – if your handbook/ inside filler cap says they can be used then you are good.