The mk3 engine will be designed for 0W20 to scrape the last 0.001% of MPG, personally I use 5W30 in our jazz MK2 and Civic. Just got some Castrol Magnatec 5w30 A5 stop-start oil from Tesco at £17 for 4 litres ( half price ) apparently you can leave it is for 20,000 miles, but I won't. That article you linked to suggested changing oil every 5,000 miles, must be an old article ( or American, they seem to recommend short oil changes, must be for oil companies to sell more ). I used change oil every 10 to 12,000 ( twice a year up until a couple of years ago when I retired ), but now once a year is plenty and then some.
IMHO the oil will be ok, the 5W refers to cold viscosity to get around quickly on cold starts, the 40 refers to hot viscosity, but I doubt it will get hot enough in UK to warrant 40, but if you want to use it you may drop a few % in MPG, but it won't harm the engine. I still do no fully trust the 0W20 and thinner oils, and will not go below 5W30...especially in UK where -5deg C is considered cold.