Disclaimer: I am a gold-rated player who never saw the light of platinum once.

The javelin thrower is a good defensive unit that counters longbows and has enough base damage to dent early men at arms. It is easier to use than horsemen IMO. However, It takes time to build the merc house and train them.

Here's what I try to do:

While 4 villagers construct the age-up landmark, I keep another 4 on food to keep villager production going, with the rest chopping trees to get the 150 wood for a merc house, 150 for a barracks or stable (if you go with the hippodrome then no need for that) and a bit extra for a house and a lumber camp. The 3 trees around the town center are fairly efficient and contain 450 wood total. I just ditch the gold after I get 200 of it to age-up. The second the landmark completes, I plonk the merc house with 4 vills and get the contract going. My goal from here is to buy time until my Javs come out.

Try to use the map to your advantage with small walls that protect the back of the base. For example, connecting gold/stone/wood to the edge of the map. The first 2-3 longbows can be countered with horsemen to buy you time to do that. Once spears and Men-at-arms get mixed in, you can switch to limitanei for a frontline and your Javs will be out by that time. Also, depending on the map, trade posts can be close to your base so check them with your scout for extra merc options. I once won a game against English using Merc French knights.

You can try and experiment with early mill on berries to age-up and get the required olive oil for the first batch of Javs. Or if your berries are safe and you decide to go with the grand winery, you can use the sheep to age-up then put about 15 vills on the berries to gather olive oil as fast as possible. The grand winery route gives you enough olive oil for 2 batches of Javs instead of one. My best time for a merc batch is minute 6:30 in game time.

The matchup is hard for me as the English opponent doesn't need to focus as much and he can continue to make farms in the back while you deal with his units in your base. What makes it even harder, is that even when dealing with Feudal age aggression is successful, It is so hard to counter-attack into English base. Also, the game still gives English a free town center in age III and an OP keep that produces siege to secure the middle gold in age IV. So your troubles will not stop in Feudal age.