Well there are laws and the enforcement of laws...
A phone call to either Waters and Stanton or Martin Lynch and Son will get you advice and be able to supply what you need. Both suppliers are main dealers for all the main brands and have internet sites to browse or physical stores if you want to visit.
If you still think you might want to use a computer, the Funcube Dongle+ at £149.99 covers a huge range of frequencies and has built in filters for various bands. There is a coverage gap between 250-410MHz which rules out military airband, but otherwise this is a very good receiver that is not much bigger than a USB memory stick. Check the details and reviews on the internet. I use one of these and the performance is considerably better than the repurposed TV dongles, but I listen to a very wide range of broadcasts across all the amateur bands from short wave to UHF.
The Funcube Dongle was originally produced as part of a project to enable schools to monitor data from micro satellites as they passed overhead, so they have plenty of sensitivity and the ability to receive satellites with a very basic home made antenna. Airband is trivial by comparison.
There are alternative receivers that operate in a similar manner, above and below this price. A certain amount of fiddling with the software and setting up will inevitably be involved with all computer based receivers, but once the system is up and running, it is usually very stable. While the computer software tends to get updated fairly frequently, you can stick with whatever version works for you. There is generally no need to keep updating unless it fixes a bug that is bothering you or introduces a new feature you want.
Handheld receivers by Yeasu, Icom and Kenwood are the gold standard and priced appropriately. There is some good cheap stuff available direct from China. Again, check the internet for comments and reviews. Handheld scanners are useful because they can be used anywhere, but do need batteries replacing or charging arrangements. Most work 8-10 hours on a set of batteries or a fresh charge.
Remember, you need to be able to receive AM transmissions on airband and the frequency steps at 8.33 kHz rather than the 12.5, 25 and 50 kHz tuning steps used in other bands. Most older receivers with 12.5 kHz tuning have a sufficiently broad receiving bandwith to hear 8.33 kHz transmissions if tuned slightly above or below the exact frequency.