Buying from overseas can attract VAT and customs duties. Also the courier may charge a handling fee which is also subject to VAT.
I suggest reading
https://www.gov.uk/goods-sent-from-abroad/tax-and-duty for a useful overview of the position.
Hobbyking has three warehouses but only one in the UK.
You need to check very carefully that the item you purchase comes from the UK warehouse.
Past dealings have indicated that whilst you might start looking at just UK warehouse items the site can select stock from another warehouse.
If so you may be charged extra costs and as others have found the savings are all taken by the taxman and the courier company.
I usually try to purchase in bulk to offset postage charges but if buying from overseas you need to watch the total cost to avoid any fees.