The UK used to have a Landing Card system for all non-EU/EEA/Swiss citizens. You had to fill it out on the plane and present it at passport control. It asked basic questions but did not ask anything about criminal convictions.

The Landing Card system was abolished in 2019, with Britain trying to appear more open to the wider world as it turns it back on Europe.

Also, Heathrow and a number of other larger airports now have an e-gate system where you can scan your passport and, if there are no problems, go straight through and enter the UK without seeing a border officer. The e-gates used to be only available to UK and EU nationals, but at Heathrow they have expanded it to other "trustworthy" nationalities, including Australians: https://www.heathrow.com/arrivals/immigration-and-passports

So you could in theory arrive at Heathrow and enter the UK legally without going through a human official.

As AB2014 says above, even if you went to a human passport officer, it's extremely unlikely that they routinely ask all travellers if they have a criminal record. I have been to several non-European countries and gone through standard immigration checks, with a few basic questions about why I'm there, where I'm staying, how long I'm staying, etc., and I have never been asked about criminal records.

Thank you for not judging me. Yes I was on a register. Initially it was for 15 years. But i was removed off it after 8. These days it's for life. The Australian system under a far right government has become extraordinarily harsh. Sex offenders are now prohibited from even possessing a passport let alone travelling. I avoided the new regime as it my offence was so long ago.. 

Rest assured, no one on this forum will judge you.
I'm not familiar with the Australian system. Slightly surprised, however, that a 4 year custodial sentence for a zsexual offence didn't result in you ending up on their Register?

Thanks for that. Well unfortunately my offence was a sexual one. 15 years ago i was imprisoned for 4 years for possessing child pornography. I was a disgusting and vile person and in a completely different world. Please don't judge me, I've been judged/punished and then some.  Im not on any register but i imagine my passport could be flagged or our border force may notify UK authorities of my travel? Unsure if the risk is worth it. Would dearly loved to have travelled with my wife and kids. It was a prize win so free travel and accommodation. 

To add: UK immigration are usually more concerned with visitors who are likely to overstay their welcome or have come to work without a visa. If you are travelling with family, have return tickets, accommodation booked and can show an itinerary that makes it obvious you are tourists you will check all their boxes. Per other comments, the main risk is if you are somehow on an Interpol watchlist due to the nature of our offence.

Hard to say, really. I don't imagine they ask every person who arrives in the UK whether they have a criminal record. Unless your record is for the sort of thing that gets you onto Interpol lists, they probably wouldn't know about you. I suspect many people travel to many countries around the world without disclosing their criminal record, enjoy their stay, don't cause trouble and go home again.

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If you are to punish a man retributively you must injure him. If you are to reform him you must improve him. And men are not improved by injuries. (George Bernard Shaw)

Thanks for that.  As an Australian citizen I would not generally be required to apply for a visa. I'm wondering if I could still come through customs regardless and risk not being asked? Big risk I know but I wonder if it could work? I just want a holiday with my family, not cause mayhem and destruction. 

You're right about the four years thing, and they count suspended sentences as prison sentences (just like they do in Australian immigration). If you disclose the conviction, they will almost certainly refuse you a visa. Unlock has advice about this on their website here.

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If you are to punish a man retributively you must injure him. If you are to reform him you must improve him. And men are not improved by injuries. (George Bernard Shaw)

Hi guys, 
I'm hoping to visit the UK for a few weeks next year but I'm doubtful I'll be granted entry. I'm an Aussie who was jailed for 4 years (suspended after 16 months) in 2006. I've been going through the entry requirements and it appears that as my sentence was 4 years, entry for me is permanently prohibited. Can anyone confirm this? And does the fact I only served 16 months make any difference? I've tried contacting the relevant agencies but no one is interested in talking to me. Thanks for any advice. 
 Will