If you get your TV reception from a small local relay station (typically the station will cover just a small rural town or a valley, although there are also some in cities) there probably won’t be any digital reception for you from these until analogue is switched off. So if it’s common knowledge in your area that there’s no digital reception (ask around) you should delay buying a digital set-top box until just before analogue is switched off. If you have to buy a new TV set though (because the old one is worn out) you should get a digital one.
When analogue is switched off you might be amongst the tiny minority of unlucky people who actually will need to change your aerial. This is because some of the small relay stations will use different frequencies after analogue is switched off. If so, the new aerial will only need to be roughly the same size and cost as the old one (allowing for inflation of course!) If you have had really good analogue reception on a small aerial then a small aerial designed for the new frequencies is most likely all you will need for digital.
If you want to ‘go digital’ before analogue TV is switched (so you can get the extra channels) the best way is likely to be BBC/ITV Freesat.