The problem with the VAT and duty, is that you also get slapped with a handling charge, which is usually several pounds. So the value of the charges can come close to the value of the goods.
For goods, anything over £15 value is chargeable, if it's sent as a gift, anything over £40.
But you don't need to import anything, because you can purchase the items you need on these shores. For a bag, have a look at the Capri sun bags- the larger ones with the screw top (enjoy contents first). These are incredibly strong, and are about the right volume for a Seawolf (330ml). A lot of people fit the bags in a vented cylinder/pipe to keep the bag in a certain position and to prevent it over expanding. A piece of PVC pipe will do the job, with a couple of end caps made from sheet stock, and some strategically placed holes to allow air to escape as the bag fills.
Not sure what sort of pump you want. The Seawolf as standard uses a 6 volt electrical system. Most water pumps are designed to run on 12 volts, they'll run on lower voltages, but the pumping rate will be slower. You can get 12 volt speed 500 motors if you want to convert the drive motor.
If you're using a bag, the pump needs to be bi-directional, so that usually means a geared or peristaltic model. If using the former you will need a pinch valve to prevent backflow through the pump.
If using a peristaltic pump, it's important to fit a pressure switch or relief valve to switch the pump off if going beyond a certain pressure. This is on safety grounds as these pumps can reach very high pressure indeed, quite sufficient to wreck your boat should they become stuck on for some reason.