If you are a member of SYHA then you can access hostels in other countries too.
They have streamlined and upgraded the hostels -- but they still range from the very simple remote ones and the city ones which do offer en suite rooms as well as dorms. There are also hostels you can hire exclusively for your own gathering / wedding etc at certain times of year- some are 'big houses' and quite dramatic!
Sadly they sold Carbisdale Castle.....

There are also quite a number of Independent Hostels (Blue Guide?) now -- they can vary quite a lot but if you check some of the traveller-guide sites for comments you can work out whether they would suit you.

Don't forget the bothies (Mountain Bothies Association) - most of which are really remote and small -- but amazing places to stay.
There's a Bothy Bible just out -- doesn't feature every one, but gives you a taste of what they are about!