The worktop can be installed (almost) right up to the side of the AGA.
It's critical to ensure there is a small clearance at each side so if it's ever necessary, the cooker's top plate can be lifted up or removed for maintenance / repairs.
(Normal servicing doesn't require the top to be lifted but some jobs do.)
The AGA's top plate is chamfered so it's important to ensure the clearance is to the widest part (which is at its lowest point).
'Real Wood' Worktops
If you're planning 'real wood' worktops (usually end-grain or strips glued together) then we recommend that these are kept away from the top of the AGA.
The first 50-150mm or so next to the cooker will always be warmer than the rest of the worktops, and with wood being a natural material it has a tendency to dry out more in this area, potentially leading to 'movement' or gaps/cracks.
We'd recommend considering having a granite (or similar, different material) strip next to the AGA to act as an intermediary and keep the wood away from the AGA's warmth.
'Ordinary' chipboard, granite, corian, silestone and manufactured worktops are all normally fine to be installed up to the AGA (leaving the 2mm gap as described above).
If you're unsure of the suitability for a worktop material please check with the manufacturer. The temperature of the AGA's top plate immediately next to it is around 60-80 degrees C.