In england it may be different. I only speak from my own experiences. I find that using a pack of cards the everyone knows and probably has, gives them a small since of familiarity. Thus, not rasing that suspicon.
You don't think that a magician performing magic tricks arouses suspicion? Making cards go inside bottles, making them teleport, rise to the top of the deck, change into other cards - isn't that suspicious?
What about making coins bend, or reading thoughts?
I'd hazard to say that simply being a magician and producing a deck of cards would raise up all kinds of suspicion. If not, you're probably doing some creative, smart presentation and performance to make everything seem on the up-and-up: so why not just do that with your deck?
There's all kinds of different decks you can buy in stores. Fancy poker-peek decks with indexes on the edges, cheap cards with generic patterns on the back - Bicycle's even making green, pink, blue, and silver-backed cards, among others. You can find these at Wal-Mart, Target, grocery stores, toy stores. It shouldn't be too suspicious to find.
Maybe it's a regional thing. I'm in America, too. I have an Ellusionist Ghost deck, and everyone I've showed it too has told me they think it's pretty.
This is an argument that's been discussed for at least a few years. When I first got a few custom decks and heard all the debate, I tried things out around my hometown. I've handed the deck out to people (maybe only 6 or 7 different people, so this isn't a conclusive bit of data) both with and without saying I do magic.
Here's a pretty basic template of what I got. Note that this wasn't part of a presentation to prove the deck was legit - these were just casual back-and-forths about a deck I happened to own.
ME: (showing the deck) What do you think about this deck?
THEM: It looks cool.
ME: (spreading the cards both face up and face down) What about now?
THEM: Looks neat.
ME: (handing them the deck) Check it out.
THEM: (spreading through the cards) Where'd you get it?
ME: On-line.
One person asked where they could get one. Afterwards, I asked if they thought anything was wrong with the deck. Anything tricky?
THEM: No, why? (looking through the deck) Is something wrong with it?
Obviously not verbatim, but that's how every conversation went. These were strangers at a local hobby shop I used to frequent.
The point being, this whole thing about how custom decks inherently bring about an air of suspicion is completely false. Maybe someone will dream up deception towards it, but I argue that those same people find suspicion with all that you do.
It's presentation, pure and simple.