Correct gas pipes to use.

Yes it's safe and legal. The reason two different materials are used is due to the installation/supply volume required for the the tower block. Steel pipes are larger so are used to supply the block with gas, they then take each flat supply down to copper as the gas volume for each flat is lower. The size of pipework is all worked out depending on appliances in the flat and length of pipework etc. This is all figured out during the building process. Solder is a proven and recommended method to seal both gas and water pipes in a flat/house. Just to put your mind at ease, a gas pipe after the gas meter governor typically has 21mb +/- 2mb of working pressure (mb= millibar). Central heating working pressure can be upwards of 2 bar or 2000mb. So if solder can seal and hold a central heating pipe it has no problems sealing a gas pipe. Hope this helps.