Is this for a mk1 or mk2 Lotus Cortina?
And why would you want a pre-engaged starter? (I thought standard issue in the UK was an inertia starter?)
Anyways, if you are after a pre-engaged starter for a mk1 or mk2 Lotus Cortina, your best bet is to do some searches for a Cortina mk1 Lucas M35G starter (9 teeth bendix) on US eBay, http://www.eBay.com
as they usually pop up over there, for reasonable prices.
Or, if you are not hellbent on getting a pre-engaged Cortina mk1 Lucas M35G starter, there is another alternative, that should be easier to source in the UK.
This will be the pre-engaged Allegro Lucas M35J starter (also with a 9 teeth bendix).
This starter is similar to the 1968-70 pre-engaged Cortina mk2 xflow Lucas M35J starter (the Cortina mk2 xflow starter has a 11 teeth bendix, while the Allegro has a 9 teeth bendix).
However, the Allegro starter is slightly bigger than the Cortina mk1 starter, so on a mk1 (Lotus) Cortina it might end up being a bit close to the engine mounting?
A friend of mine is running an Allegro starter in his mk1 Cortina 1500GT, it was a bit harder to put on due to limited space, but worked fine (you can either remove the starter side engine mount, or remove the starter solenoid, then it will be easier to put in the starter).
Here is one Allegro starter currently for sale on UK eBay, see
http://www.ebay.co.uk/itm/Lucas-M35J-Starter-Motor-25209-BL-Maxi-Allegro-1-5-1-7-1750-Anti-Clock-9-Tooth-/191376645350