Ace, I don't know where you're coming from or for how long you intend to stay, but if you're from Oz, Canada, NZ or South Africa (you did say the Commonwealth) you should be able to get in without a visa for up to 6 months as a tourist, but are prohibited from working. As per my Lonely Planet guide,
The immigration authorities have always been tough, and this is unlikely to change; dress neatly when entering the country, and carry some evidence that you have sufficient funds to support yourself.* A credit card and/or an onward ticket will help.* People have been refused entry because they happened to be carrying documents (perhaps work references) that suggested they intended to work."
If you want to work and are not an EU national, you need a work permit, and if your main reason for coming is work, you must be sponsored by a British company.
Ah, but, citizens of Commonwealth countries aged between 17 and 26 can apply for a Working Holiday Entry Certificate that allows you to spend up to 2 years in the UK and take work that is "incidental to a holiday" (sounds like pubs, farms, etc.) Apply at your local UK mission (you must apply overseas) it doesn't say how long it takes. Look up the British High Commission nearest you for details.
I've never had any problems getting into the UK on a Canadian passport (nor my husband and kids on US ones), but have always had a return/onward ticket...
Also, you might want to have evidence of some kind of health insurance (either your provincial/national insurance card, a private service or temporary travellers insurance (see the travelpunk home page for suggestions about this) - I live in Lithuania and since EU accession they have got picky about people having this before entry, I imagine the UK will look for something similar, especially if you intend to stay for a longer period.