So they company has offered you 10K gbp worth of options, vesting in 3 years.
What this means is that in three years, assuming you are still with the company, you will have the right - not the obligation - to buy shares in the company at the price indicated in the option, the exercise price (aka the strike price).
You currently have been given 10K gbp worth of options - it's tricky to say how these options were valued but I guess that isn't important.
What is important is understanding what is going on - the firm that is trying to hire you is offering you an incentive, and it is up to you to work out how much that incentive is worth compared to other places.
For example - let's say this place A is offering you 20 K / year and 10K of options, while place B (paying market rates) pays 40 K / year.
In the three years, assuming no real change in salary, in place A you have 60K and the options, while elsewhere you have 120K.
That means the options are worth - or must be worth - 60K in three years for you to consider taking this offer. But that's silly - you wouldn't take the risk of the options being worth 60K to not actually make any money (why take the risk, just take the market price!).
So instead you would want, based on your tolerance for risk, to find options worth, say, 180K. So that in three years you have doubled what you would earn in the market. Not bad.
Now you want to ask the company what the expected value of the options in three years is. You can ask that, of course, but more importantly you want to find out what the growth of the company is, what the success of the company has been, and from that work out how likely it is that the options will be worth more money. Do they have graphs of revenue/customer growth (they do), do they have expected new revenue streams (one hopes so!).
Finally, let's assume the value of the options is based on the company shares being worth currently 1.01, and the option strike being 0.01 (option value of 1 gbp and you own 10,000). In three years, if the company shares go to 18.01, each option you have creates a value for you of 18 gbp. So you have your 180K gbp!
On the other hand, if the strike is 119.00, and the stock is at 120.00 (option value of 1 gbp and you own 10,000), then the stock needs to go to 137.00 for you to have your 180K.
What this means is that the strike price of the option and the current stock price are important factors for you to consider when working out what the growth trajectory of the company will have to be in order for the options to be worht some value. An 18 fold increase in a share price is unbelievably hard - but increasing it only 14% is very, very reasonable.
Finally - the company is private but its value is known. The investors and management have a very clear idea of what the company is worth - each share will have a value, and there will be some known number of shares.
Another answerer mentions liquidation preferences -
Liquidation preferences protect the investors in the company (not you, not the CEO - rather the people who front money for the company) by giving them preferential treatment in the event of a major activity - like selling the company. If the company sells for less than some amount, effectively the liquidation preference comes into play.
An example can be found here - https://www.feld.com/archives/2004/07/liquidation-preferences.html
Generally you won't be too worried about liquidation preferences, if only because it's unlikely they are going to be shared with you. In any event, they are (generally, unless your CEO is an idiot) only for under-performing companies. What you want to find out from the company is, to you, how likely it seems that the company will be able to move the stock price by whatever % it needs to within 3 years to get you the money you need to take on the risk of working at this place.