Selling sets on eBay - should I buy missing bricks and figures to complete a set?
Is it worth it to buy the missing figures - will we make back the money we spend or would it be more cost effective to sell it with the two figures missing ?

This boils down to the following:
SALE PRICE OF COMPLETE SET &gt; SALE PRICE OF INCOMPLETE SET  +  BUY PRICE OF MISSING PIECES

If that is true, you stand to profit from completing the set, barring any other expenses (like transaction fess, currency conversion fees, shipping and handling costs, etc.)
So now to find the values of these items (you have already mentioned that it costs 16 GBP to replace the figures) we can turn to Bricklink's price estimates and lots for sale to see that used, complete lots are for sale for as low as 63€, and even new, sealed sets can be gotten for 70€, so your 100 GBP price is way over the market niveau. This brings our test to this state of completion:
63€ &gt; SALE PRICE OF INCOMPLETE SET + 16 GBP

You don't specify which two of the four figures are missing, that will definitely affect the desirability of the incomplete set (for which I can't find any closer estimates), so lets find some lower and upper margins.
From Bricklink again we can see that Vader costs on average 15€, Tarkin 9€, and the generic pilot along with Sabine Wren 5€ each. So the savvy collector could buy the two missing figures (if they are interested in having them) for anywhere between 10 and 24€. This gives an approximation for the difference between the prices of a complete set and your incomplete one: the incomplete set would probably sell for about 39-53€, depending on which figures are missing.
By plugging the correct value into the inequation you can then get the answer for your question as stated, but I think the most important conclusion is to use Bricklink for such price analysis and the realization that your asking price is probably too high for most of the informed buyers.