It would be impossible to fill the slot in the worktop neatly, and the pipes will expand and contract as the boiler runs so you need to leave a gap round the pipes; plus you need to leave access to the valve so boxing them in and tiling is not a great idea.

I'd make an L shaped cover from wood to match your worktop which you can  loosely fit in place, perhaps put a couple of blocks to position it against, and magnetic catches to hold it firmly if you find it gets pushed out of position a lot.