It depends what you are comparing with.
You can do both UK or abroad cheap or expensive and there are likely to be compromises either way.
The expensive UK holidays are Center Parks, or a nice cottage in Cornwall, Lake District or other expensive location. Then you have to add on petrol, probably parking while you are there, entrance to attractions, food and drink, which is expensive in these sorts of places.
All the above will add up to a sum that would easily pay for a holiday abroad, which is what people mean when they say 'you could go abroad for that'.
If you go to the Med for example, you will almost certainly have guaranteed weather, so you probably won't be paying for indoor attractions to get out of the rain. Food and drink out is generally much cheaper than the UK, you rarely have to pay for parking and you can spend a lot of time on free activities like beach, sea, pool, relaxing in the sun with a barbecue in the evening.
Of course you could holiday in the UK for less than a cheaper Mediterranean holiday, but in order to keep it cheap, you'll have to restrict activities, be careful where you park and limit eating out much more and you'll be far less likely to spend evenings sitting outside on balconies and in gardens because you're lucky to get the weather to do that.