This is quite complicated so brace yourself for a long reply.
First off, copyright. If you created the design completely from scratch, then you own the copyright in it as an artistic work. You don't need to register anything to gain copyright, but you would need to be able to prove that you created it and when this occurred. Obviously the fact that you have registered the design in the US would assist with the 'when' part. Anyone who subsequently uses either your design or something very similar to it would have to show (convincingly) that they arrived at their design independently, otherwise their use will be an infringement of your copyright.
Design right (whether registered or unregistered) is a separate matter and does not allow for the independent creation defence. Once a design has been created, providing it is sufficiently original, it provides the rights owner with a monopoly right.
Since your design is 2-dimensional and to be found on clothing it falls into the category of surface decoration and so cannot be protected by unregistered design right, only registered design right.
You have already established your priority claim to the design by registering it in the USA. For the other person's UK registration to be valid it must be 'new and of an individual character' (see section 1B
of the Registered Designs Act 1949) otherwise the registration is invalid. There are quite a few other details involved here, for instance that your design must have been made available to the public in the UK before the registration date, so I suggest you read through the whole of the RDA 1949, to familiarise yourself with the process.
Fortunately the law (section 11ZA(4) RDA) permits you to challenge the validity of the registration by the other person on the grounds that it infringes your copyright
11ZA Grounds for invalidity of registration.
(1) The registration of a design may be declared invalid on any of the grounds mentioned in section 1A of this Act.
(2) The registration of a design may be declared invalid on the ground of the registered proprietor not being the proprietor of the design and the proprietor of the design objecting.
(3) The registration of a design involving the use of an earlier distinctive sign may be declared invalid on the ground of an objection by the holder of rights to the sign which include the right to prohibit in the United Kingdom such use of the sign.
(4) The registration of a design constituting an unauthorised use of a work protected by the law of copyright in the United Kingdom may be declared invalid on the ground of an objection by the owner of the copyright.
(5) In this section and sections 11ZB, 11ZC and 11ZE of this Act (other than section 11ZE(1)) references to the registration of a design include references to the former registration of a design; and these sections shall apply, with necessary modifications, in relation to such former registrations.
but you need to very sure that you can prove the facts relating to the copyright aspect. Alternatively you can send him a cease and desist letter on the basis of copyright infringement, which if he ignores it you could commence litigation. His registered design right will then, no doubt, be considered by the court and declared invalid, because it failed to meet the s. 1B grounds. Should you need any more detailed advice or support I suggest you contact ACID
. This part of the IPO website
explains how to challenge the registration of a design.
If you consider the other person's activities are sufficiently large-scale and damaging to your business, there may be grounds for pursuing a criminal case against them, in which case you should contact your local trading standards office, although no doubt ACID will be able to advise on this aspect too.