Not having a certificate is usually not a problem for club racing but Nationals, Europeans and Worlds are another story. However, in the upcoming Worlds 2016 boats without a certificate are very welcome to participate in the Bronze fleet.
The alternative is having your boat measured. If you browse the UKWA web pages you will find a measurer near you, If not email the UKWA secretary. But be aware that the boats are made by hand and sometimes a hull is not entirely conforming to specs. Those hulls are not thrown out, they are still good cruising or school boats but cannot receive a certificate. I remember a boat that was taken out of the mould too soon and got a much flatter bottom. It should be obvious that such a boat is easier to get in to a plane and will hence be faster and never be allowed to race, but it is still a good boat for other purposes.