Worktops for an Ikea kitchen

I've orderd an Ikea kitchen minus the worktops as we didn't like the choices at Ikea. I've been advised that when choosing worktops, they should be at least 606mm as 600mm won't allow any overhang.

When they say at least 606mm, does that mean I should be aiming for something that is 620mm for instance? If so, where is the best place for something like this, B&Q, Focus, Homebase?

I'm not too sure on this but I've a feeling that Ikea measurements are not the same as UK spec and therefore you can't mix and match.
Worktops are a standard size, you can order a custom size but the cost will be more and not many companies will offer the option. I'd try surfing the net.
Go back to Ikea and have a look at how the worktops sit on the cupboards. It's usual to have an overhang but the deeper the overhang the more chance of hitting your head on it when you stand up! The overhang will also block access to your top drawer so I wouldn't add to it.
You can mount your worktop within a wooden frame to add a few inches at the front and back, or just add a nosing that fits on to the front of a worktop (don't buy a rounded edge top if that's the case) or you can "extend" the depth of your worktop by adding a row of tiles or strip of marble along the back wall, supported on a batton, with the worktop fitting up against the "extension".

A 620mm will do nicely. I bought the units from IKEA and the worktop else where, no problem.

Easiest option is to by 650-670mm double postformed worktops and rip the back off them.

Ikea units have no space behind for pipes etc so it can be handy to set the base units off the wall and use the full width of the wider tops. Any good timber merchant will be able to order worktops for you, they will probably be 4200mm long.

Jason