If you have had no time to collect your customs certification and money at one of the customs and tax free counters on a German airport and you can give proof to this fact you are able to get the tax free cheques stamped at the embassy in Riyadh. Any tax free imbursement letter has to be presented together with the bought item. The German tax laws require the embassy to charge you for each single certification 20.00 € as a fee. That means if you have a value of tax refund less then 20.00 € the fee will consume the money you will get back.
If you bear a UK visa valid for 180 days you are not entitled for tax free shopping as the European Union law states that you have a residence permit valid for 6 months !