I don't know the species but it is not size that differentiates moths from butterflies. My copy of Garden Wildlife (pub. Collins in its "Nature Guide" series) gives the main differences as:
- Most moths are nocturnal and butterflies diurnal,
- Butterflies' antennae have a bulbous tip, those of moths are plain or feathery.
On the size of the animals, some moths are larger than the smallest butterflies.
I think that creature is a butterfly, from being active in bright sunshine and from the organ on the antenna tip. .