An FX swap is a financial instrument that involves the exchange of an agreed amount of a certain currency for another currency, and then the reversal of this transaction at a later date. In this scenario, the UK-based company is essentially engaging in an FX swap. They are initially exchanging euros for Mexican pesos, and then at a later date, they are reversing this transaction by exchanging the Mexican pesos back into euros. This is the fundamental principle of an FX swap. The company is using the FX swap as a funding mechanism for their Mexican investment, which allows them to mitigate any potential foreign exchange risk associated with their investment.

Choice A is incorrect. An outright transaction involves the exchange of two currencies at a rate agreed on the date of the contract for value or delivery at some time in the future. This scenario does not describe an outright transaction as there is no agreement on a future exchange rate.

Choice C is incorrect. Currency futures are standardized contracts to buy or sell a particular currency at a future date and price. In this scenario, there's no mention of any standardized contract to buy or sell currencies at a specific future date and price.

Choice D is incorrect. A forex forward involves an agreement to exchange specified amounts of two different currencies at a specific future date and exchange rate. The company in this scenario did not agree on any specific future date or exchange rate for converting euros into Mexican pesos and vice versa, hence it does not represent a forex forward transaction.