The traditional brass gland is still the norm for SWA cables I think - in either BW or CW types depending whether a seal to the outer sheath is required (e.g. for outdoor situations). There as some semi-plastic alternatives these days - e.g. https://www.tlc-direct.co.uk/Products/TLST20S.html but I've no personal experience of them

Where the armour doesn't need a connection or is terminated in some other way, plastic stuffing glands are sometimes used (although in situations like a TN supply to an outbuilding which is a TT island, there's usually a need for the end of the armour to be electrically accessible for testing, so often a brass gland is still used - if into a plastic enclosure).

As for sizing, I've never found any definitive standard - each cable and gland manufacturer seems to have their own slightly different way of doing things. The thread size (20, 25, 20S etc) not being consistently relatable to the cable size across manufacturers. As there also some manufacturing tolerances in the overall cable diameter,  I've even seen some suggestions that the actual cable diameter should be measured on site to select the correct gland size. Most packs of glands come with a table of gland sizes to cable sizes.

"Earthing nuts" (e.g. https://www.tlc-direct.co.uk/Products/PRPEN20.html) can be very useful where terminating into an enclosure wall that can't be relied upon for PE continuity to the gland (e.g. plastic boxes).

    - Andy.

Has the OP got a particular application in mind?

This is a very general question, with lots of possible answers.

Just to keep the ball rolling, another popular gland type in some industries is the EW (double compression seal). Commonly available in brass, aluminium or stainless steel; with it without Nickel plating, and typically a choice of seal materials such as neoprene or silicone.

Thanks for your valuable reply!!

If you could suggest some good hole saw cutter for making cable entry holes in electrical panels, it would really help a lot. 

I have found CK hole cutters to be very satisfactory - they are available in 20 mm, 25 mm, and 32 mm.

If you could suggest some good hole saw cutter for making cable entry holes in electrical panels

For plastic enclosures, I much prefer a stepped cone cutter over a hole saw - much quicker, cleaner and one cutter for multiple sized holes.

   - Andy.

A lot of suppliers have guidance on selection in their catalogues. For example

https://www.cablecraft.co.uk/cablecraft-catalogue

As Andy says BW and CW are the go-to for SWA in the UK but being aware of other options is advisable particularly if you work in a specialist field. Wiring styles in other countries vary.

In terms of sizing again there are fairly standard tables in catalogues which suggest that there is a fixed relationship between gland size and cable spec, and for smaller commonly used cables experience suggests that is not unreasonable for day-to-day application.

BUT beware that there is no fixed link and for larger or non-standard cables you need to consider the cable dimensions in detail in relation to the glands.

Do not forget to check the manufacturing tolerances over the nominal size: It is not uncommon with larger cables for the tolerance to span two gland sizes, with the upshot that you need to measure the acutal cable on receipt and then order glands, particularly if you buy from a distributor who will themselves be buying from more than one factory. Yes, this can be a pain, which needs to be manged somehow, the solution depending on your project. But I have seen glands which have been tightened all the way up without sealing on the cable, and glands which won't go on securely, despite "correct" on-paper selection based on nominal size.

The same applies to cleats, as an aside, some styles of which are even less tolerant.