Watford is one of the easier London commuter towns to reach from central London. The fast trains from Euston to Watford Junction take 16 to 20 minutes and run every few minutes; the slower London Overground from Euston via Wembley adds about 15 minutes. Watford Tube (on the Metropolitan line out of Baker Street) takes longer but lands you closer to Cassiobury Park.
By road the M1 (junction 5) and M25 (junction 19/20) both serve Watford directly — handy for the Warner Bros. Studio Tour, which has a large car park. From Heathrow, the M25 takes about 30 minutes outside rush hour.
For the Studio Tour specifically there are dedicated coach transfers from London Victoria, and a shuttle bus from Watford Junction (£3 return) that meets every tour slot.