most insurance companies will offer you less than its value and most people accept this as final.
What you need to do is go throught autotrader or similar and find examples of costings for cars just like yours was. Look for same spec, similar mileage, same colour, service history etc to get an idea of a replacement cost.
This cost should be matched (or very close) to the offer then given by your insurers. If the first offer isnt acceptable then give them all the info you have to prove what it will cost you to replace.
Many people accept first offer as they are either unaware of what they should get or that they are desperate for a replacemtn car and dont want to spend time waiting for the payout.
good luck