Date Marriage Allowance take-up from policy introduction (#623) (#1698)

* Date Marriage Allowance take-up from 2015-04-06 (#623)

The take-up parameter previously started at 2000-01-01, decades before the
allowance existed (the allowance was inserted by Finance Act 2014 Sch. 2
and came into force 6 April 2015). Move the effective date to that
introduction date, document the 0.5 value as the post-2019 steady state
(per HoC Library SN00870, which notes outturn costs were consistently
below forecast in the early years because take-up ramped up slowly), and
add the HMRC Marriage Allowance statistics page as a third reference.

* Add changelog entry for #623 take-up date fix

Author: vahid-ahmadi

changelog.d/623-takeup-start-date.md

@@ -0,0 +1 @@
+- Date the Marriage Allowance take-up rate parameter from 2015-04-06 (when the allowance came into force) rather than 2000-01-01, document that 0.5 is the post-2019 steady-state value, and link the HMRC Marriage Allowance statistics page.

policyengine_uk/parameters/gov/hmrc/income_tax/allowances/marriage_allowance/takeup_rate.yaml

@@ -1,11 +1,13 @@
-description: Share of eligible couples who claim Marriage Allowance. HMRC outturn shows ~2.1m claimants vs ~4.2m eligible couples (~50% take-up). The matching parameter in policyengine-uk-data (`parameters/take_up/marriage_allowance.yaml`) is the load-bearing one for microsimulation, applied stochastically when populating `would_claim_marriage_allowance` per person; this file should be kept in sync.
+description: Share of eligible couples who claim Marriage Allowance. HMRC outturn shows ~2.1m claimants vs ~4.2m eligible couples (~50% take-up) at steady state; take-up was materially lower in the first few years after the allowance was introduced in April 2015 (HMRC outturn costs ran consistently below original forecast for this reason). 0.5 is therefore the post-2019 steady-state rate. The matching parameter in policyengine-uk-data (`parameters/take_up/marriage_allowance.yaml`) is the load-bearing one for microsimulation, applied stochastically when populating `would_claim_marriage_allowance` per person; this file should be kept in sync.
 values:
-  2000-01-01: 0.5
+  2015-04-06: 0.5
 metadata:
   unit: /1
   label: Marriage Allowance take-up rate
   reference:
-    - title: Income Tax Act 2007 s. 55B
+    - title: Income Tax Act 2007 s. 55B (inserted by Finance Act 2014 Sch. 2)
       href: https://www.legislation.gov.uk/ukpga/2007/3/section/55B
     - title: House of Commons Library — Income tax allowances for married couples (SN00870)
-      href: https://commonslibrary.parliament.uk/research-briefings/sn00870/
+      href: https://commonslibrary.parliament.uk/research-briefings/sn00870/
+    - title: HMRC — Marriage Allowance statistics
+      href: https://www.gov.uk/government/statistics/marriage-allowance-statistics