In the UK is it illegal to use pepper spray as self defence on an intruder in your own home or property?

The police advice in the UK is that you can use 'appropriate methods' to defend yourself and your property. You do not have to think hard about the appropriateness in the heat of the moment, but the force must not be excessive. In those terms, it is up to you to think whether it is good to carry a pepper spray on the off-chance that you may be attacked. You might have to justify in court your action of using the pepper spray.