Paisley is close to Glasgow Airport, which has numerous routes to the London airports (the cheapest would probably be EasyJet to Luton (NW London), Stansted (NE London) or Gatwick (South London). British Airways also fly to Heathrow, which is nearer the city, and Gatwick, but would be more expensive.) An alternative is taking the train from your local Paisley station to Glasgow Central and getting a train from there to London Euston on the West Coast Main Line via Carlisle and Preston. Look up Virgin Trains for this.
On foot
from where
shanksys pony.
About 412 miles on the M6
Cycle or hitch-hike
When I was in the service, one way was intercity bus ( I don' t remember what they call it in Great Britain) but, for me, it was and I assume still is the cheapest way to 'get about,' as they say.
Get a bus/coach from Victoria Coach Station.
If it's Paisley, Glasgow then the quickest way is a flight to Glasgow International Airport (not Prestwick airport). The airport is a 5 minute drive from Paisley Town Centre.
london heathrow
By bus.
Take the greyhound bus unless you can get a good deal on the train.
Strap it on your back and start walking.