Ignition Key Difficult to get in the lock.

[Rich in Vancouver]

I work as a locksmith (dont do cars though) and im not a big fan of WD40.
Its mostly solvent and leaves very little behind in the way of lubrication. It is very good for cleaning parts though.
I use a product called Tri-Flow which contains Teflon. It is amazing on any mechanical parts but I am not sure if it is available in the UK.
That said, with the electronics in modern car ignitions I am leery of squirting liquids straight into the lock. I would spray the key then insert it to try and get a small amount into the cylinder.
Not a big fan of graphite either as it is quite messy. A supplier once gave me a bag of white graphite powder and it worked well without staining everything it touched, but while driving home with the bag on the console it struck me that if I got pulled over by the police I would have a lot of explaining to do!