This is a difficult question to answer as it is very fact sensitive. Redundancy has a specific legal definition which will be the starting point in any case. The definition can be found in s139 of the Employment Rights Act 1996. This states that an employee is dismissed by reason of redundancy if the dismissal is wholly or mainly attributable to:
(a) the fact that his employer has ceased or intends to cease to
(i) carry on the business for the purposes of which the employee was employed by him, or
(ii) carry on that business in the place where the employee was so employed, or
(b) the fact that the requirements of that business
(i) for employees to carry out work of a particular kind, or
(ii) for employees to carry out work of a particular kind in the place where the employee was employed by the employer, have ceased or diminished or are expected to cease or diminish.
So firstly you must be an employee and most commonly a redundancy occurs when the place where you work closes or your role disappears. If someone takes your place then it is not likely to be a redundancy situation. If you are unsure whether you are redundant or have doubts that you have been treated fairly you should seek advice.