Wow. Questions from somebody outside N. America, that we can actually answer. (Often, we get asked questions that require that we know what's available in your market . . . and we don't -- we just know it's NOT what we have here.)

Anyhow:

1. Sodium bicarbonate IS "Total Alkalinity Increaser", aka "sodium acid carbonate" aka "sodium hydrogen carbonate".

2. The solution to foam is to sanitize sufficiently, and drain and refill frequently. In general, if your spa is foamy and you've maintained adequate continuous halogen (chlorine or bromine) levels, you need to replace the water. (Many people in the pool industry won't get in a foamy spa!) Chlorine + filtration will NOT remove everything that spa users bodies add to the water.

3. Trisodium phosphate - Na3PO4 -- CAS# 7601-54-9. In the US, it's often sold in hardware and home repair supply stores as a cleaner. I don't know if it's available that way in the UK. Directions here: http://www.poolforum.com/pf2/showthread.php?18044

4. Anti-scale - again, regular drain and replace will usually solve the problem. I don't know about water supplies in the UK; well water from karst areas in the UK would tend to be high in calcium, but surface waters usually are not.

Assuming you have a fiberglass or plastic spa and that stainless steel or bronze are the only metals exposed to the water, THEN
=> never add calcium
=> only use alkalinity if your pH is unstable. If you can, replace baking soda with borax (sodium tetraborate, sold in the detergent section in US stores) + whatever acid you have, to buffer your pH levels.
=> if your spa is not exposed constantly to sunlight, use household bleach to chlorinate. If it is exposed, use bleach + dichlor powder or granules.