The parcel should have an customs declaration on the outside detailing contents and value. If it gets "stopped" HMCE will base taxes on that plus the post office/parcel force will add a handling fee. If the parcel is marked as gift the duty is not charged on items less than £36 or if the item is not a gift and the value is less than £22. Also if the parcel is more than the above value duty is waived if less than £7.
In the past when I have brought a lot of items on ebay it seemed the bigger the parcel the more likely it got "stung" IMHO.
HMCE have the right to open a parcel to value it's contents and check for illegal items. Always best to get the sender to complete a customs form stuck on the outside of the parcel.
10 Shirts and 4 jeans will be over the gift allowance so I expect you will get caught for 17.5% VAT unless the clothes are children's clothes. Not sure if there is any import duty as this depends on the item and country, but I read somewhere that gifts below £290 are charged at 3.5%.
Also you will be charged tax on the post cost.