You can order a cert from the GRO (nowadays called the Identity and Passport Service):
http://www.gro.gov.uk/gro/content/certificates/
The site has all the info about how to order online, but as far as I know you can also order by post or phone so shout if you want to know about that and I'll have a look for the info!
The easiest way to order from the GRO is if you have found the references on their BMD indexes, as for instance you say you found Thomas Bell and Annie Dolman's marriage Jan-Mar 1928. You go on the GRO website and fill in all the information on the online form, though in the case of a marriage you only need to put the name of one person, either the bride or the groom, you don't have to put both. So you would order a marriage certificate for 1928, tick "yes" for "Is the GRO reference known"? and enter the name Thomas Bell, quarter Mar 1928, district W Derby, volume 8b, page 373, all taken from the GRO index, enter your payment details etc and then you will get an estimated despatch date (when it will be sent to you).
You can also order from the local register office, and in this particular case it will be just as easy as ordering from the GRO because Lancashire have their own ordering system accessed via their website:
http://www.lancashirebmd.org.uk/
If you search for the marriage on there then when you have found it, it shows a reference number next to it - not the same as the GRO reference numbers - and some areas will allow you to order online, but for the Liverpool area you have to print off an application form and post it in. To get the printable form, just click on the reference number again and it will come up, mostly filled in for you.
Incidentally, Lancashire BMD shows the groom's name as Thomas J Bell and says they were married at Christ Church, Everton. The other way you could get to look at their marriage certificate would be, if you can easily get to the Liverpool Record Office and see if they hold the Christ Church, Everton marriages for 1928; if so then you could look through to find the marriage certificate and view it there and get a copy. Record offices charge different prices from each other for copies so I don't know if it would be cheaper than from the register office.
If I were you I would leave his birth certificate until you have his marriage certificate with father's name and occupation, then when you have that info, post it on here and we will help you find the correct references for his birth certificate, as it could be very tricky to order a birth certificate for someone with a name like Thomas Bell without the full references.