You have done everything that is required of you.
If you wish to be taxed on your share, s837 ITA 2007 requires you to make a declaration to that effect and give notice to HMRC that a declaration has been made. The notice must be in the form required by HMRC (the Form 17)
https://www.legislation.gov.uk/ukpga/2007/3/section/837
(3)The declaration has effect only if notice of it is given to an officer of Revenue and Customs—
(a)in such form and manner as the Commissioners for Her Majesty's Revenue and Customs may prescribe, and
(b)within the period of 60 days beginning with the date of the declaration.
s115 TMA 1970 confirms that notice may given by post.
https://www.legislation.gov.uk/ukpga/1970/9/section/115
(2)Any notice or other document to be given, sent, served or delivered under the Taxes Acts may be served by post, ...
And the Interpretation Act 1978 covers service by post. As long as you can show that you sent the form to the correct address and properly paid for, it is deemed to have been delivered in the ordinary course of post: in the case of signed for the ordinary course is next day.
https://www.legislation.gov.uk/ukpga/1978/30/section/7
7References to service by post.
Where an Act authorises or requires any document to be served by post (whether the expression “serve” or the expression “give” or “send” or any other expression is used) then, unless the contrary intention appears, the service is deemed to be effected by properly addressing, pre-paying and posting a letter containing the document and, unless the contrary is proved, to have been effected at the time at which the letter would be delivered in the ordinary course of post.
If the beneficial ownership changes in any way, the form 17 becomes ineffective. The default 50/50 will apply from the date of change unless a new notice is given.