Firstly, It will help the third world to develop. The current policy of foreign aid only benefits the corrupt dictators of third world countries. This policy would offer real help by increasing the GDP and decreasing the unemployment of these countries, allowing them to grow naturally and without having to depend on foreign aid.
Secondly, It will decrease our net CO2 emissions and create a more pleasant environment to live in. We will not be able to simply cut our CO2 emissions without any negative economic consequences, the increase in forest cover will, for obvious reasons, in effect reduce our net CO2 emissions. The increase in plantations in the foreign countries shall also decrease their net CO2 contribution as well. It shall also create a more pleasant environment to live due to the large increase in forest cover and woodland for tourists, locals and people from the rest of the UK to visit.
Thirdly, It will increase our food security. We are currently relying very heavily on foreign countries to import food from. We cannot carry on like this as food is not simply another good or import, we rely on it very heavily.
Fourthly, it will increase affordable housing. The "Land Economy" publication by the Adam Smith Institute claimed that 950,000 houses could be built on 0.3% of the countries farm land, I have extrapolated my figure from this. An extra 3.8 million houses over however long this project shall take (I am under no impression it shall take much longer than this remainder of this Parliament) is a Godsend.
Fifthly, it is cheap. If housing could be done by the private sector it would not only cost nothing, it would increase tax revenues from increase activity in the construction industry and VAT on building materials. The cost for land in other countries ought not to be much as it is relatively unused, and many countries shall be desperate for foreign investment such as the Congo and Zimbabwe. True, the cost of setting up farms would be a few million for each farm, but overall not much in comparison to the government budget as a whole, and could probably fit easily in the scope of the foreign aid budget, set to reach 0.7% of GDP soon. Planting forests may be rather expensive though, however as it is such a large project there may be significant media interest and therefore it ought to be relatively simply to encourage rich tycoons and individuals or even entire companies to sponsor it and receive naming rights in exchange for the forests or sections of forest they plant.