If it has substantial or even any parts missing which is nothing good working order, then they should tell you that it’s got substantial parts missing and they should do so in the advertisement. If they do not do so, you are entitled to assume that it is in good working order and complete
As it stands now, they are in breach of the provisions of the Supply of Goods and Services Act in that the goods are not as described, they are not fit for purpose and they are not of satisfactory quality.
You are entitled to return the goods for a full refund.
Although you are entitled to return the goods for a full refund, you may find that it difficult to get the money out of them and unless they actually accept they have misled you. The fact that it may have said on the paperwork that you got at the time of delivery, that there are parts missing is absolutely immaterial.
If they will refund your money, then your only alternative is to take them to the Small Claims Court. You need to write to them if they will not refund you and tell them that unless they refund you no later than the next 10 days, you will take them to the Small Claims Court without further notice.
Here is the link to the Small Claims Court for you.
Can I clarify anything for you?