Firstly considering the "out of the country" plea I am afraid that it will not wash IF (and of course it is a BIG IF) HMRC trace you and you have no assets in the UK. I am afraid that without a crystal ball I cannot say if they would pursue. EU debts are pursued for less than £10,000 but of course it is easier to get that sort of debt within the EU. All I can say is that it is feasible that they could seek to recover the debt IF it existed. Do we know if there will be a debt?
Of course if you had an asset left in the UK then of course any established debt could be attached to that asset or you could be threatened with legal action against your UK asset/property.
I might be tempted to answer them with details of the person(s) involved and say that all transactions were undertaken based on their advice and you suggest that HMRC contact them as you have so far been unable to get a response yourself.
I am afraid that I am not an expert in Pension funds but my unresearched gut feeling would be that if you received benefits from a pension fund by means of arrangements whereby they would be taxable in the UK if your realised them before the pension conditions were satisfied HMRC would be looking to tax them if you received them by a circuitous route and arrangements while you were in and resident in the UK.
As regards XXXXX XXXXX to the helpline are you saying that somebody said don't return it or you will pay tax on it and you decided to omit it knowing that tax was due on it? Do you have a transcript of such advice and the name, date and time of the call?
As regards XXXXX XXXXX I don't imagine it would be very difficult to establish who you had the pension with if they know that there was a transfer. Of course if you adopted what was a marketed avoidance scheme then there would have been a requirement for the scheme providers (if they were resident in the UK) to advise HMRC of what had taken place and by whom. Whether of course they would cooperate with HMRC in dealing with the matter further is another question.
It could be that the information available to HMRC was what inspired them to send you a tax return to complete to see if you would return the income as taxable.
Watch about your stance "if they have nothing to go on" - clearly if they know that there has been a transfer and you have omitted it from your tax return they could make what is termed a jeopardy assessment (if they think that there is a chance you are going to do a runner - your response that transfer was made in anticipation of emigration) and assess what they consider is the correct tax liability. So you see that you could get into a worse situation if they didn't know the amount and guessed /estimated that it was more than the £50K that came to you.
Presumably you could "assist" HMRC by pointing them to the website of the people that gave you the guidance and transactions (although they may already know).
I can only say it is possible as I had a couple of clients that came to me for help after they had been in Oz a couple of years and HMRC were making matters difficult having tracked them down and raised assessments on them. It depends if you are brazen enough to think that you could get off without paying your dues.
I assume that you are talking about your form P85 being filed to claim back any taxes and to establish your intention to emigrate permanently or indefinitely from the UK. I could not recommend any such action but of course there is no taxes clearance needed to leave the UK - why not I ask myself?
You might also look at the following site to see if potentially your transfer would be ok http://www.qrops-pension-transfer.co.uk/qrops-who-qualifies although remember there is nothing better then a suitably qualified tax adviser reviewing all paperwork and documents and facts. Unfortunately that cannot take place on a forum such as this which can only provide generic observations based on the information provided.
As you will see the Q&A on the website highlighted suggests that it could still be ok if you were leaving the UK within 12 months..... potentially you could fall foul of this then given that you presumably got your transfer in 2009 or 2010?