If you have the cash (and the planning permission) by far the best way to add value is to increase the floor space, by either converting the attic or extending into the garden. Work that adds an extra bedroom will typically add 12% to the value of the property, so a £20,000 outlay will easily add at least this much value.
If this is beyond your budget, you should focus on increasing the space through clever storage solutions. In a bedroom, for example, by investing in clever under-bed storage, adding high shelves, and investing in taller wardrobes, you can turn a cramped single room into a spacious one - or convert a single into a double.