You do not have to buy an entire Bitcoin to own cryptocurrency. Bitcoin and other cryptocurrencies are flexible and accessible to everyone.
Similar to GBP, which can be broken down into pennies, Bitcoin can also be broken down into fractions. Unlike fiat, however, Bitcoin can be broken down into 8 decimal places - the smallest unit being 0.00000001, known as a 'Satoshi'.
You are free to purchase as much Bitcoin as you feel comfortable with.