You absolutely do not need to have any kind of 'fussy' ceremony.
In England and Wales, marriage is a verbal contract, so you will need to speak to each other. But if you want the absolute no frills version, your local registration office can do it for £56 (there is also a charge for a certificate and to give notice). It will be the two of you, two witnesses and two registrars in an office. There isn't a ceremony and it isn't held in a fancy room. You can literally say the two legal declarations to each other in the presence of your witnesses and registrars and hey ho, you are legally married. No party, food or fancy clothes required.
Civil Partnership is not a verbal contract, therefore if you so desire, your local registration office can do this for you, again for £56. To form a Civil Partnership you do not even have to speak to each other. Just agree to the written statement about forming a Civil Partnership and then sign, in the presence of your witnesses and the registrar (only one for a Civil Partnership).
However, both do form legally binding contracts with responsibilities as well as rights, so if you don't want to share your assets with your partner in the event of a break up, clearly neither of these options work, but as others have said, it's all about choices. We can't have it all.