You will need a structural engineer for the beam. A designer with CAD skills can advise on layout and draw the whole of the design for you in 3d, which will give you a very good representation of the space and 'flow' of the rooms. 3d drawings are much better at demonstrating this, as you will see the varying heights of units etc.that can't be seen in a 2d floor-plan. To add a WC you will need to consider where the soil stack is currently located.
You will need Building Control for the beam and the WC.

You will need to get a structural engineer to tell you what size and gauge rsj you require.

A structural engineer's drawings will suffice for the steel, but do think about Building Control and Party Wall Awards if you are doing much to a wall you share with your neighbour. For the bathroom, it's likely an architect could draw this up, but not a structural engineer, however you might just want to agree the design with your builder.

You will need a building warrant for both the beam and WC installation. Whether you need both structural engineer and architect will depend on the level of experience/ knowledge of the building regulations the structural engineers has.