Can not see the import tax being that much, should be a small package and depending how they mark it up, should not be that much.
Import VAT @ 20% is payable on anything over £15
One at $35 plus $29 postage will be £22.36 + £18.52,
So £4.47 import VAT would be payable, plus a courier fee. £22.99 before the fee
10 @ $29 plus $20 will be $290 £185.25 + £12.78
(There will also be customs duty payable as its over £135, but I believe it will work out below £9 and therefore waived) Import VAT at £37.05
Courier fees I have seen can vary between £8 and £20
Worst case scenario for 10 should be £185.25 + £12.78 paid to the supplier (£198.03)
Then £37.05 import VAT and £20 courier fees
Overall = £255.08
Divide by 10 = £25.55 each
As you have 5 buyers at the moment then you are looking back at £50 each if you have to top up the numbers, which puts you back to the territority of the original single import
Remember you will also have to take into account any costs repackaging and sending them on to each buyer
Also consider arranging a group deal through a UK retailer so they do the importing