On top of the other suggestions, ebay sold listings and a book like this are your biggest assets - https://www.amazon.co.uk/Collectors-Coins-Decimal-Issues-Kingdom/dp/0901170402/ref=asc_df_0901170402/?tag=googshopuk-21&linkCode=df0&hvadid=535908754549&hvpos=&hvnetw=g&hvrand=8432947887022540944&hvpone=&hvptwo=&hvq...
That book covers UK coins, but won’t have things like the Hercules coins which I think are Gibraltar. The prices will be conservative, but it’ll show what has value above face value.
Bear in mind if you’re going to sell, the easiest solution (sell in one lot) will get you least money. Piecing it out will net you more, but will cost you time.
Try and get more than one opinion before you make any decisions.
These pair cover most British coins from George III onwards.