My choice to go with affirming rather than swearing was about 30 years ago. I remember an event here in the States when I said I would affirm. The clerk who was assisting me looked strangely at me. I’ve been to many clerks in the UK and never had that very suspicious dubious look given for at any time. Affirming has always seemed a more truthful or honest choice for me, but I probably think too much about such things.