I was told by my accountant that having a personal plate on your van was tax deductible. Not on your car though. Almost every van that I see around here has a personal plate so there must be some incentive.
It could be possible, if the number plate could be proved to help advertise the business, ie by service you are providing or by company name.
It is debatable though and could be hard to prove. Again, depends on the value.
Perhaps £300 may go and be accepted, but £5,000 could well be thrown out.
Its one of those expenses, that does not have a definite answer. To be on the safe side, I would say no, if you really need to loose some money through your business, go and buy some assets, like a new RO, or some water fed poles. Or spend on advertising.
If you really want to get a plate, then check with your local customs and revenue, not your accountant, and get a reference number for it.
Don't forget, everyone is solely responsible for their own tax, even if your accountant tells you to do something / or advises, the end result is always on you.