Rather than "transitory", I believe the term used most often in UK legislation is "transitional". For example, if you do a google search, limiting your search exclusively to UK government websites (".gov.uk"), there are 21,100 hits for "transitional provisions" vs. only 521 hits for "transitory provisions."

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Note added at 3 hrs (2006-12-06 10:05:34 GMT)
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From L.B. Curzon, Dictionary of Law, 6th ed. (Pearson/Longman, 2002)

Structure of Act--Constituent elements of a statute, including: long title; preamble; enacting words; short title; principal, subsidiary, administrative and transitional provisions..."



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Note added at 4 hrs (2006-12-06 10:21:14 GMT)
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In the context used in your text, "disposición" does not refer to a "regulation", but rather to the provisions found at the end of a piece of legislation. Several of the references provided are, in my opinion, mistranslations from Spanish, and you will find that in the UK, US, Canada, Australia, etc. these clauses placed at the end of a piece of legislation are always called "provisions". Here are examples from those 4 countries:

Land Registry : The Land Registration Act 2002 (Transitional Provisions)
This Order makes transitional provisions in addition to the transitional provisions contained in Schedule 12 to the Land Registration Act 2002. ...
www.landregistry.gov.uk/legislation/lract2002/rules/tp1/

U.S. Copyright Office - Copyright Law
Transitional provisions; 914. International transitional provisions ... 1In 1984, the Semiconductor Chip Protection Act amended title 17 of the United ...
www.copyright.gov/title17/92chap9.html

Federal Accountability Act - Text of Bill C-2 1 / 16
Public Servants Disclosure Protection Act. 38. Federal Courts Act. Canada Elections Act. Amendments to Act. 39-59. Amendments. Transitional Provisions ...
www.faa-lfi.gc.ca/faa-lfi/faa-lfi01_e.asp

BANKING (TRANSITIONAL PROVISIONS) ACT 1959
BANKING (TRANSITIONAL PROVISIONS) ACT 1959. TABLE OF PROVISIONS Long Title PART I--PRELIMINARY 1. Short title see Note 1] 2. Commencement [see Note 1] 4. ...
bar.austlii.edu.au/au/legis/cth/consol_act/bpa1959358/







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Note added at 4 hrs (2006-12-06 10:50:06 GMT)
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Here are definitions of "disposición adicional" and "disposición transitoria" (Source "Curso de Técnica Legislativa Gretel", Madrid, Centro de Estudios Constitucionales, 1989, pp. 159-60):

--Contenido de las DISPOSICIONES ADICIONALES: los régimenes jurídicos especiales que no puedan situarse en el texto articulado; los mandatos y authorizaciones de contenido no normativo, y los preceptos residuales que no puedan colocarse en ningún otro lugar en la Ley.

--Contenido de las DISPOSICIONES TRANSITORIAS: los preceptos que regulan de forma autónoma situaciones jurídicas previas o pendientes (derecho trasitorio material); declaraciones de la pervivencia de las ley antigua; declaración de la aplicación inmediata de la ley nueva; regulación de modo autónomo y provisional de situaciones jurídicas nuevas; declaración de la pervivencia provisional de la ley antigua para regular situaciones jurídicas nuevas, etc.