For an office, even in nice oak worktops – as in kitchen worktops – are bit too shallow for a desk.
A good way to get a good desktop for cheap is to checkout the sick furniture corner in Ikea – the bit near the checkout where the shop-soiled bits are – the usually have a rack of big veneered and laminate boards.
You can get oak veneered MDF, 18mm thick and matching edge strip that you can iron on once you’ve cut to size. Prob about £40 – £50/sheet from memory. You might want to use an edge banding trimmer and end trimmer with that though and the two would set you back another £30, although you could manage without the end trimmer.
Good quality (A grade) birch ply is nice too and the cut edge is handsome so you don’t need to dress it with anything. Use something like ‘Sanding Sealer’ instead of normal vanish – its less yellowy and retains the pale quality of the birch better