I think you'll be fine. Here is the law:
(11) A person shall not cause or permit:
(a) a group of small balloons exceeding 1000 in number to be simultaneously released at a single site wholly or partly within the aerodrome traffic zone of a notified aerodrome during the notified operating hours of that aerodrome unless that person has given to the CAA not less than 28 days previous notice in writing of the release;
(b) a group of small balloons exceeding 2000 but not exceeding 10000 in number to be simultaneously released at a single site:
(i) within airspace notified for the purposes of this sub-paragraph; or
(ii) within the aerodrome traffic zone of a notified aerodrome during the notified operating hours of that aerodrome;
without the permission of the CAA;
(c) a group of small balloons greater than 10000 in number to be simultaneously released at a single site except with the permission of the CAA.