If you want a painted mask, then by all means, go ahead and paint designs on your fencing mask. Exercise your creativeness and to personalise your equipment! It is not the organisation’s intention or desire to stifle artistic tendencies.
However, designs painted onto masks should not be offensive or inappropriate (for example, no adult material or objectionable content). Furthermore, your painted design should not breach any copyright or other intellectual property, nor should it break any UK laws (such as those regarding heraldry). If your choice of design does cause problems or legal challenges for any reason, then the Academy cannot and will not be held responsible, and you will need to resolve the situation yourself.
You may want to investigate British copyright laws, along with the exceptions for “fair use” and “public domain”, which tend not to mean what people often think they do. A good place to begin your studies would be the UK Copyright Service website, especially their pages about “Understanding Fair Use” and “Using the work of others”. Copyright law is difficult and complicated, so you should seek professional advice if you have specific questions about your proposed design or artistic activities.
As a general guideline, if your design is tasteful and could not cause anyone to object on legal or moral grounds, then it should be perfectly fine.