Do you know the height dimension of the shed? 2.5m is fairly tall for a shed.
For info., you can erect a fence on your boundary without permission that is 2.0m tall, so if the shed was 2.0m or less then it wouldn’t be an issue.
If it’s taller than that, then unfortunately it might be down to local quirks in planning and if you’re on a relatively new estate you might even need permission from the original builder. Easiest route, is to know how big a shed it is, work out how much it would stick above the garden wall, if it’s more than 2.0metres tall and could be seen by the neighbour, just knock on their door, say hi, and tell them your plan.
If you go on the planning portal, it should ask you to put your post code in and direct you to your local planners advice page. But there is info down the planning page I’ve linked that might be sufficient for you.