Depends on where you live. In the US, motorcycles don't have to comply with yearly emissions testing like full size vehicles do so it's perfectly legal in the US to remove the cats. Since your quote was in pounds and not dollars, I'm not sure what the laws are across the pond.
I think this is where we can say we do live in a great country because technically if the powers that be want to push it they do have the laws behind them. I think you'd have to be a real butt to get written for exhaust modifications and don't think a LEO does not have the right or written law.
Our laws here in the US could be interpreted in different ways but there is wording that can be used to stop any and all modifications that can be tied to the science of emissions.
Here's some interesting reading...
Motorcycle Emissions Regs Examined - Motorcycle USA
Example:
What's Legal?
If all this regulation chafes at the neck, fear not, for the perceived oppression isn't as bad as it seems. Or at least is nothing new, because the "kit-bike" and "custom-motorcycle" provisions are in fact relaxations of the standing 25-year-old laws.
That's right, it turns out that while current and upcoming emissions requirements are more stringent than their 1980 forbear, the exceptions for "Custom Motorcycles" and "Motorcycle Kits" were just that, exceptions. One life-time Kit Bike and multiple Custom Motorcycles exempt from EPA emissions are more than those under the standing 1980 regulations, which permitted zero, zilch, nada, none whatsoever.
The average custom motorcycle consumer, however, need not worry about using that literal once-in-a-lifetime Kit Bike exemption. The vast majority of SVMs already source powerplants from the major engine manus, like S&S, TP Performance, or one of the others who are in the process of seeking EPA approval (don't worry about this year, though - remember, SVMs have until 2008 for Tier 1 compliance).
Still angry and want something to gripe about? Well, how about this: It is still explicitly illegal to modify a motorcycle in a way that makes it become non-compliant with EPA emissions requirements, and it has been since 1980. It's right there in the Clean Air Act itself; if you want to read it with your own eyes check out section 203(a). The gist is it is illegal to tamper with the motorcycle in any way that makes it non-compliant with emissions standards.
What is tampering you might ask? Well, like any law, the definition is open to interpretation. Doomsday scenarios by some envision tampering to include the seeming unrelated task of putting on the wrong sized tires. Other more optimistic interpretations limit tampering to the direct removal or disabling of emission regulation components, like catalytic converters and O2 sensors.
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