Weighing up cost against convenience
Heathrow is further from Milton Keynes than Luton, so public transport options usually involve at least one change, adding time and hassle with luggage. A direct taxi avoids all of that and can be shared between passengers to bring the per-person cost down. For early or late flights when public transport is limited, a taxi is often the only practical option.
Compare local taxi operators
Browse airport transfer firms in Milton Keynes for fixed Heathrow prices before you decide.