For FEDEX, DHL, and UPS only:
Internal Revenue Service
3651 South Interregional Hwy 35
Austin, Texas
78741
U.S.A.
Attn: Receipt and Processing
Contact Telephone: (512) 460-7944
Price for FEDEX last year was £38.38 for a 50 page return. Price may be more expensive for a 300 page return (?).
A return mailed in the AM of one day will reach the IRS in Austin by 3:00 PM the following day (UK time). It will be signed for, and a digital copy of the signature may be found on the FEDEX site.
Peace of mind comes at a price.