When a company is dissolved, someone takes over the company's asserts, often the creditors, or they may be sold for what they will bring. Failing anyone else, it may be the crown.
Assuming that the copyright was in fact owned by the company (and not by someone else and merely licensed to the company), someone owns it, (just as someone owns the physical property that the company had) but it may be hard to find out who. It is even possible that the owner is not aware of the copyright.
As for Person A, that depends on the details of A's contract or agreement with the company. A may be a co-owner (unlikely). A may have rights to use the code to some extent. Or A may have no more rights than I do (that is, none at all).
According to "Ownership of copyright works " an official UK government web page:
Where a literary, dramatic, musical or artistic work, or a film, is made by an employee in the course of his employment, his employer is the first owner of any copyright in the work (subject to any agreement to the contrary). The expression “in the course of employment” is not defined by the Act but in settling disputes the courts have typically had to decide whether the employee was working under a ‘contract of service’ (eg as an employee) or a ‘contract for services’ (eg as a freelancer or independent contractor).
Where a person works under a ‘contract for services’ he will usually retain copyright in any works he produces, unless there is a contractual agreement to the contrary.
Thus if person A was acting as an employee of the company, the copyright would automatically be owned by the company, in the absence of any written agreement between A and the company.
The page goes on to state:
When you ask or commission another person or organisation to create a copyright work for you, the first legal owner of copyright is the person or organisation that created the work and not you the commissioner, unless you otherwise agree it in writing.
However, in some circumstances, for example when copyright is not dealt with in the contract to commission the work, courts may be willing to find that there is an implied licence allowing the commissioner to use the work for the purpose for which it was commissioned. This does not necessarily result in a transfer of ownership. Instead, the commissioner of the work may only get a limited non-exclusive licence. ...
Thus if A was acting as an employee in creating or co-creating the code, and there was no written agreement on the matter between A and the company, A would have no rights to the code, and the copyright would pass to whoever purchased or was awarded it after the company dissolved. Such a copyright might well have been included in a purchase of "all other assets" (or some such language) and no explicit or specific mention of it been made.
The situation in the US would be basically similar. Work by an employee within he scope of employment is a "work-made-for-hire" (WFH) under US copyright law( See 107 USC 101), and the employer is legally the author (which is not true in UK law), unless there is a written agreement to the contrary. The natural person(s) who in fact created the wok have no rights unless an agreement grants such rights. On dissolution the copyright does not end, nor is it transferred to the creator, but passes to whoever bought it, or bought or was awarded the general assets of the company (such as in a bankruptcy proceeding).