It is illegal to release 20 or more gas-inflated balloons at or about the same time.
The balloons should not have any attachments. On-the-spot fines can be given and penalties are much higher if over 100 balloons are released.
This ruling comes under the Protection of the Environment Operations Act 1997 No 156 (section 146E). There are a number of exceptions to the offences, for example where the balloons are released for scientific purposes.
Any released balloons, at best, become litter. They may also end up in the ocean where they are ingested by marine animals. The balloons, along with any ribbons or plastic disks attached, can harm the animals by blocking their airways or becoming lodged in their intestines.