Most local auction houses are happy to give
verbal estimates of the value of items free of charge (especially if you hint that you might be thinking of selling them). However if your insurer requires written valuations you'll need to pay a fee. Further, you'll probably have to approach an independent High Street jeweller (rather than an auction house), taking care to note any specific requirements of your insurer (such as needing to use the services of a Member, or Fellow, of the Institute of Registered Valuers).
A bit of googling suggests that a £50 valuation fee is not uncommon but also that some valuers will assess up to three items for a single fee (possibly with a percentage of each item's value added on top).
Example:
https://www.hattonjewels.com/jewellery_valuations.html
Members of the Institute of Registered Valuers:
https://www.naj.co.uk/find-an-irv?SearchUnits=m&InclusiveSearch=false&SearchRadius=25