Not if doing so violates the UK anti-discrimination laws. The issue here is that making a "support gay marriage" emblem does not violate the rights of the baker/t-shirt maker/printer. Refusing to provide a service to one demographic while happily providing that service to others is discrimination.
Ashers are happy to make wedding cakes, just not ones with the word gay on them. The courts decided (rightfully) that doing so was a direct discrimination against the gay community.
So you'd agree with the hypothetical example presented above? Including the mohammed picture?
also 'Refusing to provide a service to one demographic while happily providing that service to others is discrimination.' they're not refusing to provide a service in a general sense to that demographic - it is the political message they're opposed to