I would always shop around to get the best quote for any type of policy. Without reading the full policy and comparing it to what is in the market, I could not advise you whether it was worth it or not. It depends on the cover it provides i.e. does it cover any defect? What I will say is that you are covered under EU law under what was the sale of goods and services act, and is now the 2015 Consumer Act for up to six years for any product (not just vehicles) if you bought the vehicle new from them.
BMW will have also tested your car over many millions of miles, they should not have designed it to fail. If buying a warranty gives you the peace of mind that you wish for, buy one. If it is not for that purpose but because you do fear something will break, I would look to see what the common failures are and see whether they are more or less than the cost of the warranty. Being less than six years old, if anything did go wrong with it, if it has been serviced by MINI you may find any issues will get resolved by them as a "gesture of goodwill".