Rymans and WH Smiths sell draughting pens of varying thicknesses. Most decent art shops do, also. Self-adhesive labels are good for corrections - again Rymans or Smiths.
I bought a Chart Correction kit from Kelvin Huges many, many years ago and am still using it. Had all sorts of goodies in it, incuding a high quality ink pen, stencils (excellent, with buoy shapes, and chart symbols etc) and lots more. I'm sure I've seen the same thing advertised recently, so a call to KH may give you the details/price.
Otherwise, as previously posted, Smiths or any large office/drawing supplies outfit will provide all you need.
I just use a ball-point pen for corrections: not as neat but quick & easy and does the job. Print the blocks onto ordinary paper and use a contact adhesive such as PrittStick. Remember to print blocks using 1:1 ratio (no stretching) and use a printer that has waterproof ink.
