how about continue boxing all the way to the window and use it to house concealed shower pipes and taps also. adding a niche for shampoo and soaps / candles. to have a mirror cabinets above the sink and toilet and under sink cabinet. also fill the gap on the other side of the bath adding a cabinet for laundry / cleaning stuff.

A carpenter would make that existing basin fit into a purpose made cabinet and then get a worktop.

Have one bespoke made by a carpenter/joiner. Not expensive and an easy job

Obviously a wall hung unit would show more of the floor so would give the illusion of more space but a floor standing unit would conceal the boxed in pipes, the sink pipes and would give you more storage. A bathroom fitter would be able to fit the unit neatly up against the box. But you would need to choose a unit that can be modified easily.