It's a nominal order. If there is no assets then they can't pay it, simple as. However, if assets are later discovered, the court can revisit that order and further confiscate.
Didn't live a lavish lifestyle? They've spent an additional ~£40k a year, well over the median income, aside from the wages they earned legitimately. Unless there's evidence of expensive living, holidays etc the investigators just haven't found the money and need to look harder.
