Depends on the airport - for Heathrow airport, I have been using Wheely, which is an app service like Uber.
They offer transfers to Heathrow airport from SW12 for only £40.
The AMAZING thing is that Wheely is a premium car service - all their cars are either Mercedes C-class or E-class cars. Which beats hands down all the other minicab car services for comfort, luxury, and driver professionalism. I've never had a bad Wheely service to or from Heathrow so far, and I've taken more than a dozen rides.
You can prebook through the app. There is a first booking discount with a referral code - if you need one, do send me a PM or email!
I would use them for Gatwick and City as well, but unfortunately their pricing is much higher than other services for these two airports...