If I go to GRO in Edinburgh and book a seat is the 1851 census there for me to use yet?
Yes. It has been available there for decades.
However you can also get at the 1851 census free of charge, in variousl libraries. Which library depends on where you are interested in.
Edinburgh Central Library has the counties of Shetland, Orkney, Caithness. Sutherland, Ross and Cromarty, Inverness, Nairn, Fife, Stirling, Clackmannan, West Lothian (aka Linlithgowshire), Midlothian (aka Edinburghshire), East Lothian (aka Haddingtonshire), Berwick, Selkirk, Roxburgh, Peebles, Dumfries, Kirkcudbright, Wigtown and Ayr.
The
Mitchell Library in Glasgow has the counties of Lanark, Bute, Renfrew, Dunbarton, Stirling, Argyll and Ayr.
The
Tay Valley Family History Society in Dundee has the counties of Angus (aka Forfarshire), Fife, Kincardine, Perth and Kinross.
Aberdeen Public Library and the
Aberdeen and North East Scotland Family History Society have the counties of Kincardine, Aberdeen, Banff and Moray (aka Elginshire).
The above list is not exhaustive - there are many more libraries and family history societies who have some census microfilms (e.g
Inverness Public Library has the counties of Shetland, Orkney, Caithness. Sutherland, Ross and Cromarty, Inverness and Nairn), but these four between them cover the whole of Scotland's 33 counties.
So save your paid-for time at the GRO in New Register House for BMD certificates, and use the free libraries for census (and OPRs).