I am not a lawyer, but I am guessing that if Facebook blocks giphy usage in the UK there is very little the UK regulator can do to them. I see that as the likeliest scenario as they won’t want to sell giphy.
This will only make the regulator look foolish. The average person won’t understand why Facebook owning giphy is a problem (it’s not a social network) but they will be annoyed when they lose access to giphy in slack, WhatsApp and other platforms that use it in.