Hire car/taxi
There are taxi services available and you can book on-line. They are not black cab services, just private hire.
We have never used these, but we know people who have and said it was expensive but convenient. Just, one time the car was a bit smelly.
Coach
You can get a coach service towards London. As you might expect, this is a lower cost option.
It is very easy to find the coach departure point. Coaches go to Victoria Coach Station, with multiple stops in north London on the way, and other destinations like Stratford.
Of course, you can go to other places, but we want London.
The probability is you won’t have heavy bags, and, if you’re not in a hurry and don’t mind delays due to traffic, this may be a good choice.