This is in fact very simple when you know what BA is doing. It all comes down to the intricacies of revenue management. I will attempt to explain it step by step.

I expect that fares from France are much lower than from the UK.

BA is surely expecting very high demand during this peak period and believes it can sell all its seats.

BA's revenue management system will assign a so-called bid price to all seats on the flights. This bid price is the minimum fare that a seat can be sold for and is based on the history of average fares sold on a given origin-destination route in a point of sale (typically country) market.

Seats will only be available if the fares in the market are greater than the bid price. In more detail, a given fare class will only be available if the expected average revenue based on history received in that fare class is greater than the bid price.

BA's RM system knows that fares sold in France are generally low, probably lower in fact than this bid price.

Accordingly BA believes it can sell all the seats from higher fare points of sale and there is no availability from France.

BA also believes that the normal fare paid by passengers buying in the UK is higher than France, higher in fact than the bid price, so seats are made available for sale in the UK.

France fares however can be purchased in the UK, so you can buy WT in the UK and not France using French fares (priced in pounds) at UK availability. This is an old trick and can be used to get cheaper fares if the lower fare class is available in one country and not in another.

Finally BA's UK call centre will have the same access to inventory as opodo.co.uk so you can call them and buy a ticket in WT from them.