I think you are approaching this from the wrong angle IMHO.
Presumably the first question to answer is, in the event of an accident in an aircraft carrying upto 4 people, what is the potential size of third party claim.
In this increasingly litigious world, £5M would not be inappropriate, although many people carry far less.
The marginal cost of another £1M third party cover is relatively low for the peace of mind it brings.
More directly related to your question, £3M will satisfy the vast majority of large airports that you will be flying into across Europe. Where there are specific requirements such as in Denmark, Germany and on RAF airfields, you'll find that many of the policies on offer provide this cover as part and parcel of the overall plan.