2 lbs / 0.9 kg sugar in 1 US gallon / 3.8 liters produces a SG of 1.090. Your recipe calls for 6.6 lbs / 3 kg in 8 liters of water, which is 3.3 lbs / 1.5 kg in 4 liters. This produces a SG above 1.135, not including whatever the fruit contributes. This is a rich enough environment the yeast may not start. Shorting the water will make the SG higher. I recommend for beginners to not start with the SG above 1.100 so that your likelihood of getting a good fermentation is higher.
A recipe suggestion -- do not blindly add a set amount of sugar. Add 0.9 kg for each 4 liters, add the fruit, stir well, and check the SG. If it's between 1.085 and 1.100, you're fine. Don't add more sugar than will bring the SG above 1.100.
Shorting the water could push the SG up too much. I lean against it for that reason. If you were making 10% ABV wines it would work ok, but the sugar level of your recipe is way higher.
I have a couple of ideas:
1) freeze half the berries and make the second batch after the first is in secondary storage. If you have the freezer space, this is a no-cost solution. Actually, freezing the berries breaks down the fruit for better extraction, so it may be a good idea to freeze them first.
2) get another, preferably larger, primary fermenter. Are Rubbermaid Brutes available in the UK? They're food grade and a lot of us in the USA use them. One in the 37 to 45 liter range (10-12 gallons) is good for everything you're likely to make. I have a pair of 32 gallon / 121 liter Brutes for fermenting larger batches of grapes, and am considering buying one or two 10-12 gallon Brutes for smaller batches.