You simply can't do this.

As soon as you buy the property in UK or move back or show any business or personal connection to the country the tax office will deem your move to Germany as tax evasion and will tax your capital gains. If you don't declare your gains (which you are saying you won't) you would likely end up in court charged with tax evasion which is a criminal offence.

You need to clearly understand that you can't evade UK CGT by simply moving out of the country for 6months or even a year then moving back. Any move abroad must be seen to be a complete and permanent severance with you home country. A bank account, property, even a club membership or phone contract still remaining in the UK in your name can mean you are not classed as living abroad by the tax office. If you return to stay or have any business involvement with the UK within say 10 years (there are no hard and fast rules each case is examined individually and even after 20 years you could be considered to be a tax evader) you will become liable for CGT on your gains made abroad.

So you either pay CGT in the UK or you leave for good. There is no option to return. If you have benefited from the UK then maybe you should simply pay your CGT? In the year you need to hold your BTC in Germany before you can sell to be locally CGT-free BTC could de-value by 20% making the whole exercise pointless. In fact you may then decide to move back to the UK and end up paying CGT on the lesser gain so effectively having paid CGT twice!!

Read this to see how seriously HMRC/CPS take this issue and the resources directed to wards uncovering tax evasion.
http://www.cps.gov.uk/news/articles/prosecuting_tax_evasion/