High Speed 1 (HS1), formerly known as the Channel Tunnel Rail Link (CTRL), is a 108-kilometre (67 mi) high-speed railway line running from London through Kent to the British end of the Channel Tunnel.   It was built in two stages. The first section between the tunnel and Fawkham Junction in north Kent opened in September 2003, cutting London–Paris journey times by 21 minutes to 2 hours 35 minutes, and London–Brussels to 2 hours 20 minutes. On 14 November 2007, commercial services began over the whole of the new HS1 line. The redeveloped St Pancras International station became the new London terminus for all Eurostar services.  The completion of High Speed 1 has brought the British part of Eurostar's route up to the same standards as the French and Belgian high-speed lines. Non-stop journey times were reduced by a further 20 minutes to 2 hours 15 minutes for London–Paris and 1 hour 51 minutes for London–Brussels.  
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The Pinnacle 90 would probably NOT fit due to the SICT head cushions ( inches wide).
The Frontier 90 is 19 inches wide – which is quite similar to the width of your current Marathon and Roundabout seats ( inches wide).
So, if your current set up suggests that you have space for an extra 1/2 inch per seat, I would recommend the Frontier 90 seat.
If your current set up suggests that you do not have space for the extra 1/2 inch per seat, you should consider the Diono Radian RXT – at 17 inches wide, it is the narrowest harness booster car seat on the market today.