An RX1 (Application to enter a restriction) is the form itself.
Form K is a standard form of wording (not an actual form like you fill in) for the restriction which would go on the title register of the property, there are different standard wordings for different restrictions on a title register. Form K says:
Form K (Charging order affecting beneficial interest – certificate required)
"No disposition of the
[choose whichever bulleted clause is appropriate]
•
registered estate, other than a disposition by the proprietor of any registered charge registered before the entry of this restriction,
•
registered charge dated [date] referred to above, other than a disposition by the proprietor of any registered sub-charge of that charge registered before the entry of this restriction,
is to be registered without a certificate signed by the applicant for registration or their conveyancer that written notice of the disposition was given to [name of person with the benefit of the charging order] at [address for service], being the person with the benefit of [an interim or a final] charging order on the beneficial interest of [name of judgment debtor] made by the [name of court] on [date] (Court reference [insert reference])".
(completed as appropriate)
You can see them in Appendix C here:
http://www.landregist...des/practice-guide-19
Don't worry if most of it is a bit technical, it's most used by practitioners.
I'd send a certified copy of the charging order, not the original (a solicitor should be able to do this quite cheaply) to lessen the risk of it going missing.
I'd say that the Land Registry should be able to advise on the right form and restriction to use generally as it's more procedural than legal advice.
Hope that helps.