Recently the Home Office have started allowing for different names in two different passports if the law of the country of origin is strict and does not allow for the same rules as the UK.

Here’s the link to the policy (exception noted on page 7): Use of names in passports.

Exceptions

6.5 There are individual categories of applicant who may experience restrictions on their ability to meet the identity requirements of one name for all official purposes. These are primarily people who have dual nationality and who hold, have held or can obtain in the future a passport issued by another country.

6.6 Subject to the applicant being able to satisfy the following requirements, a passport may be issued in the name requested even where it differs from the name on the passport issued by another country. The following categories may be given exceptional consideration:

i. The law in the applicant’s country of origin restricts or prevents a change of name. Where there is such a restriction, the applicant will be required to provide evidence from their country of origin that a change of name is not permissible;
ii. The country of origin does not permit dual nationality meaning alignment of documentation will not be possible, the applicant will be required to provide confirmation from the country of origin that their citizenship has been cancelled;
iii. Where there is a requirement for the person to travel to their country of origin in order to change their name, and due to exceptional circumstances in their country of origin, they would be placed at a high level of risk;
iv. The applicant has changed their name as a result of a change of gender and the law in the applicant’s country of origin does not recognise or permit a change of gender;

6.7 In the case of points (i) and (iii) above, a (British) passport may be issued and an observation placed in the passport saying: “The holder has a [country] passport, number [ ] issued on [date] in the name of [ ]. This passport is due to expire on [date].”

6.8 Consideration should be given to issuing a restricted validity passport on point (iii).This should be done in conjunction with the Operational Delivery Standards and Guidance Team who will consult with the Foreign and Commonwealth Office.

Spanish nationals

In light of this, the Spanish consulate now issues a certificate explaining this law in English so you can present it to the HO. There is a fee of £32.75 to pay for this service.

Link

On a sample British passport of a Spanish child which shows the single British surname and on the page opposite (the one reserved for observations), there is following note: “The holder also has a Spanish passport issued xx/xx/2017 in the name of Firstname Surname1 Surname2 which is due to expire xx/xx/2019”.

You may want contact the consulate for more information.