The most difficult time of the year to plan for genrp! because March / April is the beginning of spring, when winter gradually changes towards summer so it is possible still have snow and ice here in England. But that is very unlikely, particularly in April. More likely the weather will be cool (compared to the Philippines!) with a mixture of sunshine and rain showers which are usually not too heavy. Usually the temperature will vary between about 3 and 15 degrees C, and there will be some rain on maybe a third of the days, but plenty of sunshine too. Best to bring a mixture of warm and light clothing therefore, plus a light raincoat or umbrella (or just buy one when you get here). Most days from April onwards people will wear fairly light clothing with some sort of coat or jacket, but also will be prepared for cold.
But it is a nice time to visit because all the spring flowers will be coming out, and in London you can always be inside on the bad days and outside on the warm and sunny days.
Feel free to ask any other questions about London - I'm sure, if not me, then someone will be able to answer them. If it's your first visit, I hope you have a great time!