WW is probably correct. If your flight had been earlier in the day, one possible route would be train to St Albans and bus from there, but your arrival time of 20:00 means you might not reach St Albans City Station until after the last bus to WGC, which is 22:39.

On the other hand, a taxi may well be cheaper to WGC from St Albans than from Luton Airport - it's a shorter journey, for a start.

A suggestion: if you think you will make the last bus from St A, go for it and if you miss it get a taxi from there.

I'd agree - a taxi is probably best bet. By taxi it's about 25 mins. Otherwise you have to get the shuttle bus to the train station, wait for the train, then wait for the bus, etc. If all goes well, I'd guess it'd take you c. 1.5 hours.

Also, the bus that goes to WGC goes through WGC but one of the hotels (Best Western) is then about a 20min walk from the town centre. You probably would need to check how close the bus goes.

Similar question on the London forum,

https://www.tripadvisor.co.uk/ShowTopic-g186338-i17-k13196503-Luton_airport_to_Welwyn_garden_city_transport-London_England.html