Is it legal?
Yes. According to the European Regulation (EU) No 379/2014 of 7th April 2014, amending Commission Regulation (EU) No 965/2012, pilots are allowed to share flights as long as the aircraft does not exceed 6 seats and has a non-complex motorisation. Furthermore, costs can only be shared, pilots are not allowed to make any profit on the flight. We have received written confirmation from the EASA, stating that flight sharing and advertising flights is totally legal, as long as the costs are fairly shared. The CAA also confirmed it is legal in a letter to us. However, it is important to note that Air Navigation Order 2016, which states that advertising a cost shared flight is not possible, has been superseded by Order 1188 General Exception E4277 which allows cost sharing in the same way EASA does. In other words, because the flight remains a private one, the pilot does not need a commercial pilot’s licence to share their costs with passengers.
You can see a sum-up about the legality of flight sharing in Europe and especially the UK
See sum-up document