I'm looking for an efficient way to reach Luton Airport from Watford Junction, during the weekend. More exactly, on a Saturday morning. It doesn't matter if it's with the train, bus or even taxi. I would like to know what's the best way taking in consideration the price/time spent. I usually used to take the following route:

Watford Junction - Kenton - Overground (around £1.80)
Kenton - Northwick Park - walk 5 minutes (around 50 calories)
Northwick Park - Finchley Road - Metropolitan (around £1.80)
Finchley Road - Luton Airport - National Express (around £11)

After my math that would be ~£15.