Do you always get caught for import duties or do some slip through .?
A few years ago it seemed to be more likely that some items would slip through. A ParcelFarce manager once told me that there are two bins for parcels at airports and it's a matter of luck whether yours going into the one that's going to be reckoned for import duties or the other which goes straight through. I imagine with improvements to process and the desire to increase tax revenues, it's much more likely to be charged than not. Bottom line is that you need to budget for the duties and fees.
This link is for the current UK Trade Tariffs which shows the import duty rates that vary depending on what it is you're importing. Most photographic gear falls into chapter 90, lenses are currently 6.7% and 35mm bodies are 4.2%, though that's a best guess as the descriptions are written in government-speak.
Note that VAT is additional to import duty on items sent from outside the EU (Brexit pending). Import duty is calculated on the value of the item including shipping costs and then VAT is calculated on the total including the import duty.
There is no import duty on items worth under £135, it's a blanket 2.5% on
gifts valued between £135 and £630 and the value from the trade tariff for everything else. I haven't tried asking for items to be sent as gifts. There is no VAT on anything worth less than £15 or gifts valued at less than £39.
Just out of interest, what are peoples experience with delivery time to your home, do UK customs really sit on it for days/weeks?
Yes. ParcelFarce's excuse is that Customs cause the delay (even when the tracking details show an item has left the Customs building a week before you received any notification from PF) and they'll send you a letter to come in and pay the duties and their ransom so they won't even complete the job they've already been paid for of delivering the parcel to your door. It's possible to pay the carrier online and request delivery of the item but much simpler to go and get it.
There is no way to pay Customs directly and avoid the ransom charge. A few years back there were some formal complaints that the carriers were charging for an unsolicited service by paying the duties up front and then levying their ransom fee on top of the duties payable, so Parliament amended the law to specifically allow carriers to charge for these unsolicited services.