How can I convert very high, fluctuating, AC voltage to 240VAC at 50Hz?
A voltage regulator (most likely from a car's alternator) then smooths
  out the voltage to as close to 240V AC as possible.


A 12 V alternator regulator will not work on 240 V AC. It will be destroyed.


  A silicon-controlled rectifier (thyristor) converts the flow to DC,
  preferably at the same voltage.


Use diodes.


  A capacitor (or more than one if required) is connected to smooth out
  the DC signal.


With poly-phase rectification no smoothing is required. One of the phases is always up.


  A 6, 12 or 18-pulse inverter then converts the DC signal back to AC to
  be fed into the grid.


What is meant by this?


  Another voltage regulator and/or transformer could be fitted just
  before the grid tie-in point, if the voltage were not yet stable
  enough at 240V.


No. You would rectify the low voltage and then convert to 3-phase 240 V AC using an inverter. This is standard on uninterruptible power supplies. 


  I presume that by generating 9-phase AC electricity, I am still able
  to 'merge' the phases such that there will be 3 bigger phases? UK
  mains electricity is naturally 3-phase and, as such, that's what will
  need to be fed into the grid.


No. You can't merge phases.


  Is a voltage regulator (I've found a supplier of 250V ones) suited for
  what I'm intending to use it for?


No. Use an inverter.


  How can I tell what the frequency of the AC current will be once it's
  inverted back from DC?


The inverter determines the frequency.


  Are there any steps that could or should be altered to make the whole
  process more efficient?


Yes. Generate with three phases and, if possible, at the correct frequency and voltage.