Seems a good as place as any to ask. I visited Kew Gardens at the weekend. I was intending using the rail 2 for 1 offer for a free Kew ticket. My journey was ELS to KWG, so a change at West Hampstead from Thameslink to the overground. Single off peak price was £6.10 single, £11.90 return. Someone joining me was going WHD to KWG only and their fare was quoted as £6.70 single, £13.40 return. Why does it cost more to travel only half the journey?
Similarly ELS to WHD is also £6.10 single, £11.90 return; ELS to KWG is also £6.10 single, £11.90 return. So the journey from WHD to KWG is effectively free, but starting at the half way point costs more.
Of course, if it hadn’t been the need for a rail ticket, contactless or debit card ELS to KWG is only £3.70 with WHD to KWG being £1.90, which at least seems logical.