Hey, you can do 2 things.
1) You can report your sim as lost/stolen here: http://giffgaff.com/support/lost
Giffgaff will then send you a replacement sim. Once you report your sim as lost/stolen, your current sim will be blocked and cannot be used. All Giffgaff sims are now hybrid, dual sims and contain a micro sim that easily pops out. So, the replacement sim you will be sent will have a pop out micro sim. All your account details and number will be restored on your replacement sim.
2) The best and quick way for you to get a micro sim is to take your current sim to a local phone shop and they will cut it down to the correct size for you. This will cost £1-£3 and then you can use your micro sim very quickly. .