Electric oven

Hello I had a currys essentials single oven in my kitchen it was wired using a 6mm 3 core grey wire from the 45a wall socket directly to the oven the new oven i have brought is from ikea same size rates at 1850w but this oven comes with 2.5mm flex already attached. What is the best way to wire this? 1st I thought remove the 2.5mm flex and wire the 6mm wire directly to oven but the holes are not big enough for 6mm wire to go in 2nd can i instal a cooker outlet box to the and wire the 6mm wire from the switch into box and 2.5 mm oven wire into box is that safe to do or will the 2.5mm wire burn as the 6mm wire is bigger 3rd 1850w is 7.7amps so should i put a plug on the cooker wire and plug it into the same socket as the 45a switch Thanks Kev